View Full Version : 7591 Alternative


jaymanaa
02-03-2008, 09:15 PM
I think this subject has come up in other threads, but never really got discussed. I'm trying to figure out what would be a good substitute for the 7591. Most of the good pentodes get excluded by heater current, but the 6L6 looks like it might work.:scratch2: I'm not the best at comparing tube data so I thought I'd run it by you guys. Any thoughts on this or other candidates? Thanks, Jay

Old1625
02-03-2008, 09:34 PM
To the best of my knowledge the original is now once again available, but the price is close to a kidney or spleen for a pair.

Back in the day when they were temporarily not available at any price I switched in 6L6s in dozens of those old units with success. The only bugaboo is that the 6L6GC envelope is quite proud of the original, which may preclude reinstalling the chassis back in its case. All but a few customers said go ahead and they'd just use it without the case.

Socket rewiring is required, as there are some differences in the basing (in more modern parlance "pinout") scheme.

sorenj07
02-03-2008, 10:01 PM
These prices don't seem too terrible... (No affiliation)

http://tubedepot.com/75917868.html

jaymanaa
02-03-2008, 10:04 PM
These prices don't seem too terrible... (No affiliation)

http://tubedepot.com/75917868.html


Yeh, I just never cared for the sound of the tube. I've got some iron from 7591 amps and was trying to figure out a tube that might sound better, with more rolling possibilities.:yes:

bricktop
02-03-2008, 10:15 PM
Yeh, I just never cared for the sound of the tube. I've got some iron from 7591 amps and was trying to figure out a tube that might sound better, with more rolling possibilities.:yes:

oooohhh, I thought you were looking for something with minimal work to stick in an existing amp that uses 7591...but in this case, you got some more options:D. Do you know the primary impedance? I would guess 6.6k, but if its closer to 7.6k, you could build an amp that uses the new 6973, I've always wanted to do that. Lots of reserve in the transformer then for ba-bass, as trannys for 7591s are usually medium to biggerish...

jrsh92
02-03-2008, 10:17 PM
KT90's. Think POWER.

jaymanaa
02-03-2008, 10:19 PM
oooohhh, I thought you were looking for something with minimal work to stick in an existing amp that uses 7591...but in this case, you got some more options:D. Do you know the primary impedance? I would guess 6.6k, but if its closer to 7.6k, you could build an amp that uses the new 6973, I've always wanted to do that. Lots of reserve in the transformer then for ba-bass, as trannys for 7591s are usually medium to biggerish...

What's this 6973? Never heard of it, but I'm very interested. I've got iron out a Fisher KX200 (no screen taps) and some on the way from a Heath 100 with screen taps. How do I go about figuring the primary Z?

jaymanaa
02-03-2008, 10:20 PM
KT90's. Think POWER.


I've got a few old style EIs I've been saving for "something".:yes:

bricktop
02-03-2008, 11:09 PM
The KT90s might be a little much for a transformer designed for the 7591.

6973s are mini power pentodes similar to the EL84 but lower distortion. Electro-harmonix makes them new now, but I haven't tried them yet. I have an old GE amp that uses them and I have RCAs, which sound great. There are no UL curves for it, so you would be breaking new ground if you went that route. Vg2max is 330V, which is not bad, but you would have to run a B+ of 330 max, preferable closer to 300V. If you ran pure pentode, it would still sound sweet, you could use B+ of 400V, screen of 300V.

http://datasheets.electron-tube.net/sheets/049/6/6973.pdf

Anyway, see what you think :thmbsp:

Bob E.
02-04-2008, 12:00 AM
What's this 6973? Never heard of it, but I'm very interested.

Very familiar tube to jukebox guys (the ones with the last stereo amps, anyway...). They were apparently RCA's last serious tube development effort before transistors started to make inroads. Low-distortion beam-power pentode in a tall 9-pin envelope (the repros that have recently come on the market are in a much shorter envelope, I remain skeptical that they match the originals at 12 watts plate dissipation). Seeburg, Wurlitzer, and AMI all used them, I think Rockola went a different path using 7868's. I think there were some guitar amps that used 6973's, as well as some Ampex amplifiers used in consoles. Before the re-issues came on the market, NOS 6973's were often fetching $70 to $90 on that auction site. Thankfully, I have a small stash of RCA's and GE's for my Seeburgs!

cademan
02-04-2008, 03:55 AM
I have info but can't get it to upload!

Chad Hauris
02-05-2008, 11:48 PM
Yes, thank goodness for the new 6973's...they have all worked great in Wurlitzer amps.
There were some Sylvania and other brand 6973's which used the shorter envelope in the original days of 6973 production so I don't think the fact that the Electro-harmonix ones are shorter than RCA's were is an indication they are less power capable.

I used 6L6GC in a Heathkit PA amp which originally used 7591. This was before new 7591's were being made. Just rewired the sockets and it worked great....not saying of course that this would work in all cases without other modifications though.
This amp had no tubes when I got it, so I also rewired the 7199 socket to take 6U8 and the 6EU7 socket to take 12AX7. The Heathkit designers must have really had a thing for "new" tubes in this amp.

mhardy6647
02-06-2008, 11:14 AM
So... I am not the only one who's not that crazy for the 7591/7868 sound...
Funnily enough, though, I think I have at least 5 amps/receivers that use 'em. Nope, make that 6.

jaymanaa
02-06-2008, 11:20 AM
So... I am not the only one who's not that crazy for the 7591/7868 sound...
Funnily enough, though, I think I have at least 5 amps/receivers that use 'em. Nope, make that 6.

Yeh, is it maybe too laid back ya think? However, I did like Terry's Sherwood after an overhaul and using EH 7868s.:music: It had lot's of crisp detail that my Fisher using 7591s seems to lack. I just wish I could find a short bottle tube that could be wired to work without changing anything else. I do have "some" 6L6s that are short, but most are too tall to get a cover over (like old1625 said).

mhardy6647
02-06-2008, 12:34 PM
Back in the days when there was no new production 7591, the 6GM5 was recommended as a replacement... pinout is different, but I am not sure it would sound much different.

I am asking the local tube gurus and I'll report back.

Steve O
02-06-2008, 01:33 PM
Back in the days when there was no new production 7591, the 6GM5 was recommended as a replacement... pinout is different, but I am not sure it would sound much different.

I am asking the local tube gurus and I'll report back.

The reason the 6GM5 could be recommended is that it's actually a 7591 internal structure placed in a special envelope with a noval 9 pin (like 6BQ5) base. The internal structure of the couple I have look EXACLY like a Westinghouse 7591 (one is even branded W too), and tester setup is same apart from pinout. Tube manual specs are same too.

Since objective specs are same, the 6GM5 is considered a plug-n-play replacement if the socket is changed or an adaptor is used (AES used to sell adaptors till 6GM5s got expensive too).

Although I can't imagine there being any sonic difference betw a 6GM5 and a 7591, I'm sure under the right circumstances some will report just that.

jaymanaa
02-06-2008, 01:43 PM
Back in the days when there was no new production 7591, the 6GM5 was recommended as a replacement... pinout is different, but I am not sure it would sound much different.

I am asking the local tube gurus and I'll report back.


I'll check my pile.:yes:

coman61
02-06-2008, 02:43 PM
So... I am not the only one who's not that crazy for the 7591/7868 sound...
Funnily enough, though, I think I have at least 5 amps/receivers that use 'em. Nope, make that 6.

Wow me too, But I been avoiding saying that for about 2 years now for fear of being tared and feathered here.

FalconEddy
02-07-2008, 12:43 PM
Yeh, is it maybe too laid back ya think? However, I did like Terry's Sherwood after an overhaul and using EH 7868s.:music: It had lot's of crisp detail that my Fisher using 7591s seems to lack.

If your not happy with the sound of those 1960's production integrated amps that use 7591 (or 7591A's), it's not only these tubes that are giving you what you're hearing.

The stereo amps designed in the 1960's were designed to offer a particular sound. Even if you restore those amps to operate as designed, you still may not like the overall sound.

You won't get changes that are leaps and bounds audibly different by simply retooling to use 7868's or 6GM5's. Initially, that's because they're basically tubes with the same specs. Second, the signal feeding them contains the same info from the same circuit path using the same electronic components as it did before.

If you really want a noticeable audible change in those amplifiers using 7591's, rework the coupling cap stage to try to get the sound you're looking for. Polyester/foil caps will generally leave it with that original vintage feel, and are usually very cost effective (CDE, Mallory). If you're looking for a tiny bit brighter sound, use metallized polyester film caps like Illinois MWR series (a very good cap and highly cost-effective). If you want it a bit brighter, use polypropylene film and foil such as Sprague 716P, Hovland Musicaps, or MultiCap PPFX-s series. Stepping up a bit more bright, you'd be using metallized polypropylene like Solen PB series, Illinois MPW series, AuriCap, MultiCap PPMFX series, (many, many others).

The Sonicap (Gen I) cap is one of the brightest on the market, but can easily be bypassed with their Gen II series which pops the mids up, and cuts the harshness without losing detail.

If your wallet allows for excellent 'designer' caps, I'd recommend Jensen or AudioNote paper and oil with copper foil. You can also get these from Angela Instruments with their own brand labeled on them; but I've heard they're made on the same production line by Jensen in Denmark. Of course, there's also the Mundorf Silver/Gold metallized polypropylene in oil; as well as a few brands of nice teflon/foil caps out there, too.

YMMV.

Oh, and be sure to connect the inner foil side to the grid side of the tube. This has a direct effect of the frequency response being passed through those coupling caps (it's a reactance/impedance issue). Some may not hear much of a difference, but it sure looks a LOT better on a scope.

. . Falcon

WopOnTour
02-08-2008, 03:15 PM
Oh, and be sure to connect the inner foil side to the grid side of the tube. This has a direct effect of the frequency response being passed through those coupling caps (it's a reactance/impedance issue). Some may not hear much of a difference, but it sure looks a LOT better on a scope.

. . Falcon
Eddie, this final statement intrigues me. What technique are you using to identify the "preferable" grid side of the capacitor??
Thanks
WOT

jaymanaa
02-08-2008, 04:43 PM
Thanks Eddie, I agree completely.:yes: I'm planning on doing a scratch build with some output iron from a Fisher that used 7591s. I guess I could try it both ways (7591, and 6L6):scratch2:

bricktop
02-08-2008, 05:10 PM
Usually, those tubes in Scott gear is preceded by a pentode-to-cathodyne phase splitter. One could try adjusting the operating point of the pentode as well to tinker with the sound. Haven't tryed it yet, but I know they usually run at low screen voltages, so one could try bumping that up, adjusting other parts to keep plate voltage the same, to see how that will affect sound. Other ways include messing with the feedback, and then it gets progressively more involved...a good set of test equipment would be needed.

FalconEddy
02-08-2008, 08:37 PM
Eddie, this final statement intrigues me. What technique are you using to identify the "preferable" grid side of the capacitor??
Thanks
WOT

Typically, I have found that the inner foil side varies in an axial design film cap (as you read it), from manufacturer to manufacturer. They were not obtained by guesswork. I contacted the manufacturer's engineering department heads in most cases.

As you read the value printed on the cap, the INNER FOIL is:

Illinois MWR series - on the right

CDE WMF series - on the right

Mallory PVC series (radial design) - the side NOT marked with a black line

Hovland Musicap - the red lead

AuriCap - the red lead

AuriCap Teflon - the red lead

Kimber Kap - the red lead (Kimber is no longer being produced)

Jupiter - on the left

AudioNote - on the left

Jensen - on the left

Sonicap (Gen I, Gen II, Platinum) - the longer lead

Dynamicap ('E' version)- the shorter lead

Mundorf Silver/Gold/Oil - either side (double winding cancels out inductance/reactance issues)

Sprague 716P - unfortunately due to the methods of production and labeling, this cannot be identified by looking at the cap.

The best way to identify the OUTSIDE foil (shield) would be to construct a simple test generator that uses the cap as a resonation determinator. (Easier said than done). Depending which side is connected to ground, you would either be able to get the signal to 'jump' to a metal resonator plate in close proximatey, or not. If you don't get much of a signal, then the shield is most likely connected to ground.

. . Falcon

PakProtector
02-10-2008, 01:04 PM
hey-Hey!!!,
On hunting substitutes for the 7591, there is one not yet offered: the 8417. Now most uses of this valve run the hick out of it and it matters if you choose the GE or the Sylvania. run to 7591 limits it shouldn't matter much. Not that the 8417 is much eaisier to source, or for that matter fit in as it is a 6L6GC-sized bottle with more heater load than a 7591...:) In terms of gain, gm and overall performance, the 8417 is a worthy bottle.
cheers,
Douglas

WopOnTour
02-10-2008, 02:18 PM
Typically, I have found that the inner foil side varies in an axial design film cap (as you read it), from manufacturer to manufacturer. They were not obtained by guesswork. I contacted the manufacturer's engineering department heads in most cases.

. . FalconExcellent data Eddie Thanks!
For years I've noticed "some" tonal differences between the installed direction of capacitors within the signal path and thus have always made sure I attempted to at least "match" the phasing between channels when I installed coupling caps.
Thanks for sharing your research.
WOT

FalconEddy
02-11-2008, 09:10 AM
Excellent data Eddie Thanks!
For years I've noticed "some" tonal differences between the installed direction of capacitors within the signal path and thus have always made sure I attempted to at least "match" the phasing between channels when I installed coupling caps.
Thanks for sharing your research.
WOT

My pleasure.

I'm still in the process of verifying a few others.

When I get the info I'll pass it along in a new thread here at AK. It will be the complete list of film caps I've researched.

. . Falcon

GordonW
02-11-2008, 09:42 AM
My pleasure.

I'm still in the process of verifying a few others.

When I get the info I'll pass it along in a new thread here at AK. It will be the complete list of film caps I've researched.

. . Falcon

May I make a request for testing the Jantzen, Solen and Bennic/Dayton caps? Not super-high-end, but used a LOT in this sort of thing, by many folks.

Also, the GE polypropylenes that Madisound has been carrying for so many years... there's LOT of those guys out there...

Thanks in advance! :thmbsp:

Regards,
Gordon.

drewbolce
02-11-2008, 10:52 AM
This is from the AikenAmps site: http://www.aikenamps.com/OutsideFoil.htm

Some of the caps FalconEddy believes are orientated are really series wound. Note that the 716P (600v) is one. Anyway here is a decent method of finding out without dissection.

What if the capacitor doesn't have a banded end?

This marking of the outside foil was very common in the "good ol' days" of electronics, but, sadly, most capacitor manufacturers nowadays do not bother to mark the outside foil, so we're left to fend for ourselves. If the capacitor has no banded end, the outside foil connection could be on either end, so there is no easy visual method to determine the best orientation of the capacitor. However, if you have access to an oscilloscope, you can do a simple test to determine which is the outside foil terminal. Set the scope up to the most sensitive vertical scale (20mV or less, preferably) and connect the scope probe across the capacitor (ground to one side of the cap, probe tip to the other). Grab the capacitor tightly with your fingers, and note the amplitude of the induced 60Hz AC signal (or 50Hz if you are on the other side of the pond). While still holding the capacitor tightly, reverse the scope leads and you should see a dramatic difference in the amplitude of the induced AC signal. The orientation with the lowest induced signal is the one you want, and the ground lead of the scope is connected to the outside foil in that position. Mark it, and connect that side of the cap to the lowest impedance point in the circuit, typically the driving source plate when used as a coupling cap, or the grounded end if used in a shunt position. If you cannot see a large enough induced AC signal by holding the capacitor between your fingers, place the capacitor on top of an AC line cord (that is plugged into the mains wall socket, of course!) instead of holding it between your fingers and you will see a larger signal on the scope. If you are new at this, start with a 0.022uF cap or thereabouts, as it is easiest to see the difference between the two orientations. The induced signal is smaller at 60Hz with larger value capacitors, and is more difficult to see on the scope.

In the case of some types of capacitors, such as ceramic disks, multi-layer ceramics, or silver micas, there is no "outside foil", because the capacitor is made of a single-layer, or stacked layers of dielectric material and conductor. The orientation of these capacitors makes no difference. Also, some higher-voltage film caps (typically the 1000VDC/450VAC and higher values, such as the Orange Drop 716P high-voltage units) use a "series-wound" technique that has two separate sections, side by side, with a common "floating" connection layer, usually at the bottom of the layer stack. These caps will have no inherent shielding either.

Proper orientation of the capacitors will make the amplifier much less susceptible to outside noise, including hum, interference from fluorescent lighting, and tendency towards oscillations or frequency-response peaks and dips due to unwanted feedback from nearby signals within the amplifier, which can affect the tone of the amplifier (and is the reason why some people claim the amp sounds different if the caps are oriented in the opposite way - if there is no accidental coupling, there will be no tonal difference, but there will still be a noise benefit gained from orienting the caps the correct way).

FalconEddy
02-11-2008, 11:04 AM
After contacting the engineering heads at both Cornell-Dubilier and Illinois Capacitor, they have stated THERE IS NO WAY TO DETERMINE THE SHIELD SIDE (by looking at them) for their film caps that have already hit the wholesale/retail market.

This is directly related to the production methods of non-inductively wound film caps, and when the labeling is placed onto the outer jacket.

They CAN (and have) produced shield labeled film caps, but that requires a minimum order for special tooling and outer jacket printing directly at the time of manufacture.

I personally disassembled a few of the WMF and WMR caps, but have been told it was simply a coincedence that they all had the shield lead on the same side. DAMN!

Of course, these were the only two brands I hadn't verified with their engineering department. DAMN THAT BERNOULLI GUY!

Apologies for any confusion.

Also, Solen and Dayton film caps' shield could be either end, unless otherwise marked.

The Jantzen Standard Z-Cap, MKT, and CrossCap shield side is most likely either end, but the Superior Z and Silver Z both are double wound so it shouldn't make a difference (like the Mundorf).

. . Falcon

FalconEddy
02-11-2008, 11:58 AM
Some of the caps FalconEddy believes are orientated are really series wound. Note that the 716P (600v) is one.

Sorry, but that is not correct.

SBE manufactured 715P/716P series film caps rated at 600Vdc and lower do not have a series-section designed winding. The series wound film caps start at 800Vdc and up.

Other film caps that I listed as being oriented are not series wound. These are basic extended foil single wound film caps, and the shield ends are denoted with a black plastic jacketed lead.

. . Falcon

drewbolce
02-11-2008, 12:41 PM
Sorry, but that is not correct.

SBE manufactured 715P/716P series film caps rated at 600Vdc and lower do not have a series-section designed winding. The series wound film caps start at 800Vdc and up.

. . Falcon

You are quite right, I stand corrected. I always thought the series construction started at 600v, it is actually 800v as you pointed out.

While looking this up I came across this application note which gives SBE's opinion on the matter, interesting reading at the least.

bricktop
02-11-2008, 02:22 PM
wow! jaymanaa, looks what you started!:D Did you settle on what tube you're gonna use with those trannys?

jaymanaa
02-11-2008, 03:42 PM
I know, there are some great responses here, and they all seen very interesting.:scratch2: I'm going to compile a list of the numbers mentioned and go up to my storage unit and see if I can find any of them. Tubes on hand seems to dictate a lot of my building lately. That frees up money for iron and stuff.:D Do you have any favorites from the last couple pages. The 8417 that Pak mentions is interesting to me for sure. I was sort of hoping for something with an anode cap that had been made specifically for TV use, but hey, you can't have everything. So............whatcha think?

bricktop
02-11-2008, 05:07 PM
I know, there are some great responses here, and they all seen very interesting.:scratch2: I'm going to compile a list of the numbers mentioned and go up to my storage unit and see if I can find any of them. Tubes on hand seems to dictate a lot of my building lately. That frees up money for iron and stuff.:D Do you have any favorites from the last couple pages. The 8417 that Pak mentions is interesting to me for sure. I was sort of hoping for something with an anode cap that had been made specifically for TV use, but hey, you can't have everything. So............whatcha think?

Well, i'm partial to the 6973, but you won't get a whole lot of power out of a pair, and you'll have to invest in some. The 8417 is nice too, but expensive and hard to come by. I think a lot of bogen PAs and quicksilver amps chewed most of them up over the years.

You mentioned tv tubes, got any of the following?:
6DQ5 (24W Pa) -plate cap
6AV5 (15+W Pa) -regular octal
6DQ6 (15W Pa) - plate cap
6CB5 (23W Pa) - plate cap

They all run at low screen voltages, so you can't run ultralinear, but that doesn't mean they won't sound good. (Bogen db230a uses 6av5s with regulated screens - sounds great) I haven't tried anything but the 6AV5a (indirectly), but maybe some people around here have? (or you could potentially be the first)

FalconEddy
02-11-2008, 06:23 PM
I know, there are some great responses here, and they all seen very interesting.:scratch2:

Jay, sorry for hijacking this thread with all that film cap info.

. . Falcon

jaymanaa
02-11-2008, 07:22 PM
Jay, sorry for hijacking this thread with all that film cap info.

. . Falcon

Hell Eddie, I thought that was some of the best stuff in the thread dude.:yes:

I've got a slew of 6DQ6s and always thought they looked cool. Got any rough ideas for a circuit? Maybe something with an octal driver tube?:scratch2:

bricktop
02-11-2008, 08:16 PM
I've got a slew of 6DQ6s and always thought they looked cool. Got any rough ideas for a circuit? Maybe something with an octal driver tube?:scratch2:

See if you can figure out the output transformer impedance and power ratings, then we can go from there. What were they from? What kind of octal driver tubes do you have plenty of? You could potentially make this amp without buying anything, depending on your parts stash. big bonus:thmbsp:

edit: just thought of this: if you have any 6CL6s around, they make great driver tubes. You could do a LTP with a constant current sink tail. (like the bogen amp i redid) Very linear!

jaymanaa
02-11-2008, 08:39 PM
See if you can figure out the output transformer impedance and power ratings, then we can go from there. What were they from? What kind of octal driver tubes do you have plenty of? You could potentially make this amp without buying anything, depending on your parts stash. big bonus:thmbsp:

edit: just thought of this: if you have any 6CL6s around, they make great driver tubes. You could do a LTP with a constant current sink tail. (like the bogen amp i redid) Very linear!


I have one pair that came out of a Fisher X200 that was rated for 35 a channel, and another pair that came out of a Heathkit that used 7591s. I'll try to figure out the impedance. I did it one time by applying a small amount of voltage, but I'd have to find my notes (Pak told me how to do it). As far as octal drivers, I have tons of 6SN7 and 6SL7. I also have a large selection of large power iron to choose from, as well as big chokes and stuff. What do you think about SS rectification? I like it in the amps I have that use it. I'll do a search for the 6CL6's, but I should have some. What were they primarily used for. audio or TV?

bricktop
02-11-2008, 08:58 PM
I have one pair that came out of a Fisher X200 that was rated for 35 a channel, and another pair that came out of a Heathkit that used 7591s. I'll try to figure out the impedance. I did it one time by applying a small amount of voltage, but I'd have to find my notes (Pak told me how to do it). As far as octal drivers, I have tons of 6SN7 and 6SL7. I also have a large selection of large power iron to choose from, as well as big chokes and stuff. What do you think about SS rectification? I like it in the amps I have that use it. I'll do a search for the 6CL6's, but I should have some. What were they primarily used for. audio or TV?

You can do the same LTP configuration with triodes, then you don't have to worry about screen supplies too. It will also take less heater current with one 6SN7 than two 6CL6s. With sweep power tubes, you probly won't need a whole lot of drive voltage to the grids, so you'll have extra gain for feedback. The datasheets for 6CL6s say they are for "wideband amplifier use," so I guess it was for general purpose amplification. I think they were used in some ham equipment too as drivers.

SS rectification is fine (I prefer it, one less tube to worry about sourcing for the life of the amp). I think it has a bad stigma form people trying to implement it into amps that originally used tube rectification. When done right, it can sound just as good as tubes (if your into that 'sag' sort of thing). One thing it doesn't feature is delayed turn-on or soft start, but you can integrate that with switches or a delay relay timer if you desire.

Ohighway
09-21-2008, 10:45 PM
Jaymanna, Try these. Nice sounding high quality tubes, good power potential, and dirt cheap relatively speaking. AND you get to use the Anode cap.

http://www.vacuumtubes.com/6BG6.html