Converting A Fisher 660A to 6L6 tubes !

[tekuhn]

The difference in guitar amp distortion is that the artist is CREATING the sound they want for their individual instrument. The job of our audio equipment is to REPRODUCE the sounds the artists worked hard to create, as accurately as possible. That doesn't mean that quite often I don't wish that the material was mixed differently - more hi-hat, less snare, more bass guitar, etc. Unfortunately, if I try to turn up the treble for more hi-hat after the fact, I alter the sound of every voice and instrument in that frequency range which is not what I want. In the same way, distortion in our audio equipment affects the sound of everything in the source program. I don't see how that could ever be a good thing.

 

[Gary Mattis]

The problem with the gain mismatch, is that it upsets the feedback loop, and probably won't have enough gain to be properly driven by the preamp/tone stage.

If it's OPTIMIZED everywhere (redesigned to have a proper gain structure throughout, so it doesn't run out of headroom, and the feedback is properly re-set)- then yes, sometimes low-gain stages can sound better. But, ONLY if everything is set up to work with those stages.

Regards,
Gordon.

If the tubes are operating now at around 40 to 45 ma. Relatively Hot the output should sound pretty good I would think ? I do not see any type of adjustment on the NFB being just a 8 ohm tap of the winding on the secondary of the OT . It may have a little too much headroom on the highs at lower volumes, I turn the treble down a little bit. Up loud I can run it flat or turn up the bass and treble some if I want to. I guess I need to know exactly what your meaning by headroom ? At this point I guess I would want to do a compramise for running both 6L6 and EL 34. Maybe I am not understanding why it is a gain mis match , the tubes being used are normally used with 6L6 and EL 34's all the time , so why is it a gain mis match ?

 

[GordonW]

Headroom means how much extra output level you have in the preamp, when the amp reaches clipping.

You want the amp to clip first- so that you're not amplifying preamp distortion. Anything that reduces the gain of the amp section (which changing from 7591s to 6L6s will do, if the amp isn't modded to compensate), will reduce the input sensitivity of the amp- which means the preamp has to be turned up that much higher to get the same total output level. That can, in some cases, exceed the capability of the preamp section.

Also, one of the things that insures that an amp sounds more similar at low and high volume, is the proper application of feedback. An amp with a lack of bass (which is a frequent condition, when there isn't enough feedback) will sound "thin" at low volume- but, as the volume increases, the bass will sound proportionally louder compared to the highs, due to the Fletcher-Munson effect.

The original circuit is designed around the gain of the 7591- which has about 6dB more gain than the 6L6. Substituting a 6L6 will commonly decrease the open-loop gain- and also decrease the feedback, if nothing else is changed- by close to 6dB. That can be enough to upset the tonal balance of the amp...

Regards,
Gordon.

 

[Gary Mattis]

Headroom means how much extra output level you have in the preamp, when the amp reaches clipping.

You want the amp to clip first- so that you're not amplifying preamp distortion. Anything that reduces the gain of the amp section (which changing from 7591s to 6L6s will do, if the amp isn't modded to compensate), will reduce the input sensitivity of the amp- which means the preamp has to be turned up that much higher to get the same total output level. That can, in some cases, exceed the capability of the preamp section.

Also, one of the things that insures that an amp sounds more similar at low and high volume, is the proper application of feedback. An amp with a lack of bass (which is a frequent condition, when there isn't enough feedback) will sound "thin" at low volume- but, as the volume increases, the bass will sound proportionally louder compared to the highs, due to the Fletcher-Munson effect.

The original circuit is designed around the gain of the 7591- which has about 6dB more gain than the 6L6. Substituting a 6L6 will commonly decrease the open-loop gain- and also decrease the feedback, if nothing else is changed- by close to 6dB. That can be enough to upset the tonal balance of the amp...

Regards,
Gordon.

If your calculations are correct ? Is 3.98 the actual db loss or less than 4% ? . Or are you saying it is a 25% to 30% loss 6db x4 or 3.98 ? . If there is any volume loss I do not hear it , the amp is very loud about 60 watts output - if I had to guess ! . Remember I still have one 4 ohm OT with a 9 ohm resistor fooling the NFB right now into thinking it is on the 16 ohm tap, the OT is still only 4 ohm half of the other origional OT. If 4% of pre amp/output is what 6 db represents is that really a good enough reason to redesign the pre amp section ? And if the amp is still very loud is it really an issue ? . I have no problem with the bass up loud or down quiet it seems perfect, if anything is wrong I would say that the pre amp is creating slightly too much treble at lower volume. That is the only thing I do not like , I was wondering if the 4 ohm OT was causing that to happen ?

 

[tekuhn]

With the NFB loop attached to a 4Ω tap instead of a 16Ω tap, it has less feedback, not more, so the 9Ω resistor is making the situation marginally (negligibly) worse, not better.

 

[tekuhn]

To increase the NFB in the channel connected to the 4Ω tap, remove the series 9Ω resistor you added and locate the NFB RC network - it will either be R122/C102 or R117/C100 depending on which channel this is. Replace the 3.3K resistor with a 1.2K and give it a listen.

I know this suggestion is very un-scientific, but since you are tuning this by ear why not give it a try and see what you think?

You did say you were using some cheap speakers to test this with, right? I'd hate to blow something expensive if it goes into oscillation....

 

[Gary Mattis]

N

To increase the NFB in the channel connected to the 4Ω tap, remove the series 9Ω resistor you added and locate the NFB RC network - it will either be R122/C102 or R117/C100 depending on which channel this is. Replace the 3.3K resistor with a 1.2K and give it a listen.

I know this suggestion is very un-scientific, but since you are tuning this by ear why not give it a try and see what you think?

You did say you were using some cheap speakers to test this with, right? I'd hate to blow something expensive if it goes into oscillation....

No Oscillations , 2.6k is what I ended up with, 1.2k was thin sounding and bassy, next I tried 2.3 k and it was fuller but still too bassy, I added 300 ohms to 2.6k and I think that is right for me, so a 2.5k/2.7k would be the best choice on the NFB resistors on each channel plus they each have a cap and 220 ohm on each to ground. This mod got the treble a little more under control at lower volume without making it sound thin or losing bass. Frankenstein by Edgar Winter just came on and I cranked it up pretty loud and the bass was so pure that I thought it was someone at my front door knocking on the door and the highs were clear also !. It is Amazing the two 30 watt 12 inch test speakers have taken so much abuse !

 

[tekuhn]

Awesome! Your NFB levels are still not equal between channels so expect to play with it more once you have the same OT on both sides. Also I’m sure the small cap in the feedback network will shape the sound too, but I have no idea in what way. I assume it can filter frequencies in much the same way a speaker crossover network works. There is undoubtedly a way to calculate the proper values for these components, but I don’t know how. Might be a good read sometime.

 

[Gary Mattis]

Awesome! Your NFB levels are still not equal between channels so expect to play with it more once you have the same OT on both sides. Also I’m sure the small cap in the feedback network will shape the sound too, but I have no idea in what way. I assume it can filter frequencies in much the same way a speaker crossover network works. There is undoubtedly a way to calculate the proper values for these components, but I don’t know how. Might be a good read sometime.

I think those caps are called Bright Caps and that is what they do to the sound. Not sure how they pick them ? Like You said I need to research it to find out more. I added another 300 ohms this morning to see how close I could get to the 3.3k where it was and 2.9k seems to work ok also, I think at lower volumes 2.9k sounds even better. Not going any higher with it.

 

[gadget73]

Those are selected to give proper frequency response. Often its to tame rising high frequency or help with oscillation problems. The larger it is, the less high frequency response an amp has actually. Too much and its liable to oscillate. usually selection involves square wave and stability testing to get the best response without the amp getting unstable. Thats really how most feedback stuff is done, test for performance and stability to make sure its right.

 

[Gary Mattis]

I think those caps are called Bright Caps and that is what they do to the sound. Not sure how they pick them ? Like You said need to research it to find

 

[Gary Mattis]

Found 6 EL 34 tubes in my tube stash, 2 matched Svetlana EL 34 tubes and two matched Tung Sol EL 34 B tubes, plus 2 more un matched Svetlana's. I May try putting them in later , before I buy 4 Vintage Mullards for it. I could just cheap it out with some vintage Teslas , they sound pretty good also ! . Not gonna do anything else at this point but listen to it at a reasonable sound level for awhile so that I can better access the sound quality. The problem with testing powerful amps is when a song you like comes on and you crank it up , then when you go back down your ears are all messed up !

 

[Gary Mattis]

I Put two pairs of EL 34 tubes in it last night to see how it would sound , not bad at all ! Not sure which ones I like better ? The 6L6 and EL 34's both sound good in this amp.

 

[Gary Mattis]

I found out this weekend that The Cabinet was burned up by an ex wife in the driveway and that the original OT was removed because it was shorted out, so I guess I can forget about those things at this point ! . I have some old wood laying around so I guess I will make my own cabinet for it , I made a base for it today and mounted the tuner and amp on it , next I will build 4 table leg posts up to the top of the tuner and then put another platform up top for my equalizer, then fix the dial and make some kind of face plate for the tuner and maybe some decorative metal mesh of some kind. The tuner dial is up and down so I had to mount it upright, I made a aluminum bottom cover for the tuner so I could stand it upright. Still planning it as far as all that goes and I was thinking of maybe Gold Laquer on the chassis's like they used to do on amps a long time ago. I will keep the 6L6 RCA's but most likely will get a a Quad of Vintage Mullard EL 34's for it . I saved it from the scrap pile at least and it is playing music again ! . I have two Tung Sol EL34 B and 2 Svetlana EL 34 tubes and they sound pretty good for newer tubes. I put all new tubes in it except the first tube where I had a Amperex, I changed it to a vintage Mullard instead because of the EL 34's. I like Vintage Amperex in stage 1 with 6L6 tubes and Vintage Mullard in stage 1 with EL34 tubes. That first tube seems to really make a differance !

 

[crispycircuit]

The heater winding mite not like the EL34s which draw 1.5 amps compared to 1amp (6L6 or 7591) See how hot the power transformer gets? You could burn out the heater/filiment winding.

 

[Gary Mattis]

No deals o

The heater winding mite not like the EL34s which draw 1.5 amps compared to 1amp (6L6 or 7591) See how hot the power transformer gets? You could burn out the heater/filiment winding.

I almost laughed when I read your post ! , Just How hot is too hot after 4 or 5 hours playing ? I have been asking myself that question for years ? I check it after about 15 minnute's and again after about a half hour and if I can touch it I consider it ok , is this a proper method ? I don't really know but without the actual PT specs it is all I have to go by !. Seems to me like PT's can burn up anytime in a stock tube amplifier from the heat. I do not worry about it that much anymore like I used to do , seems like they run pretty hot in good sounding amps and like the heat serves a purpose to me , cold tube amps do not usually sound as good as hot or warmed up amps.

 

[Gary Mattis]

The heater winding mite not like the EL34s which draw 1.5 amps compared to 1amp (6L6 or 7591) See how hot the power transformer gets? You could burn out the heater/filiment winding.

How hot is too hot ? Been asking myself that question for years ?

 

[kward]

May be okay if you aren't running the preamp. Otherwise two EL34's will draw about 1.2 amps more than two 6L6's or 7591's. With EL34's in the sockets, check the filament voltage. If it's pulled down below 6.3V, then it's more current than that secondary winding was engineered for.

 

[Tube Radio]

I'm Not interested in using obsolete 7591tubes or newer 7591 tubes

Obsolete in what way?

My goal was to improve the 7591 mistake that Fisher made !

So you're saying that you know more than the engineers at Fisher who designed the amp?

Thanks for the info about the console and year ! As far as the tubes go it is Just my choice, I like The Sound of The RCA Holy Grail Blackplate 6L6 tubes

What if someone were to install regular 6L6 tubes in the amp without you knowing and then had you listen to it without seeing the amp or tubes? Would you be able to hear a difference?

I'm using the Fisher analog tuner and it is solid state rectified so I already understand there a few limitations built into the amps sound, that is why I decided on using the 6L6 tubes.

What limitations?

 

[Gary Mattis]

Obsolete in what way?



So you're saying that you know more than the engineers at Fisher who designed the amp?



What if someone were to install regular 6L6 tubes in the amp without you knowing and then had you listen to it without seeing the amp or tubes? Would you be able to hear a difference?



What limitations?

Not saying I know more than Fisher's designers, I'm just saying that from my point of view that they used the wrong tubes ! , My Opinion has changed from 6L6 to using EL 34 tubes over the 6L6 sense I started this project. Both tubes sound good and I am sure the 5791's would sound good also, There are a lot of tubes that could be used but only the 6L6 and EL 34 are practical with the best choices available . Even the 6L6's have gotten kind of pricey lately just like NOS 5791 tubes, To me the EL 34 has the most to offer with lots of vintage tubes like RFT and Tesla still available. Then there are a lot of vintage Mullard EL 34's , if your willing to pay the high prices for them ! . I am not sure that I want a tube with any more detail than a EL 34 has, they seem to have gotten it about right to me , plus time seems to agree with me ! . I know what I like best and EL 34 seems to be it usually. I still like other tubes just not as much in high powered amplifiers. I started out with 6L6 in this project and ended up with EL 34's. I still like the sound of the 6L6 tubes in this amp but not as much as the EL 34's. I have a 50 watt 1951 guitar amp with 6L6WGB/5881 Tung Sol tubes and I like those tubes in that amp, but in my 75 watt guitar amp I have EL 34 tubes and I like the EL 34 amps sound even better ! . I know Fisher used 5791 tubes in 1962 for this amplifier but that was their choice not most peoples choice back in 1962 or even today for that matter ! . People still use bias ply tires on antique cars also ! But is it really the best choice to make ? My new choice EL 34 is taking a lot more heater current from the PT and that could certainly be a problem down the road if The PT gives out on me, but right now it gets a little bit hot after hours of playing but so do the OT's, it works fine and it hopefully will not become an issue. It is an old amplifier and it has an old PT. My guess is that Fisher did not use a really weak heater winding in their PT with 17 tubes !. I understand their are no guareentee's on anything vintage and sense the Fisher OT went bad and shorted out maybe Fisher uses cheap transformers or min spec units and maybe a 5791 tube shorted out and it blew The OT , maybe that is why most amp builders stay clear of 7591 tubes ?. The 3 watt resistor did not blow !. I do not really know either way what happened to the fisher OT, the transformers all run hot after a few hours on this amp ! . The amp sounds good to me now and that is all that really matters in the end , if it breaks down again then like my car I will fix it !