Voltage amp for RCA console pulls

jaymanaa

RIP 1961-2018
Howdy Folks, From doing a search on this question (and a PM), I think there are a few guys around (me included) who would like to know the best way to add a voltage amp tube to the RCA 177 series of console amps. It seems that the amps are really hard to drive when you try to use them as stand alones. The one I have uses 6CG7s in front of 6BQ5s. I subbed in a 6DJ8 and a Russian 6N1P for the 6CG7 to get a bit more gain and it helped, but I'm just sure the 6BQ5s are not getting driven to full output. Any suggestions? or has anyone ever actually added another tube or two to one of these? The chassis has plenty of room, and the power supply seems plenty hefty without the preamp/tuner load. Thanks in advance for any help on this one, Jay
 
Are you trying for more power output before clipping (in this case before rounding), or are you looking for more gain?

In push-pull configuration a pair of 6BQ5's should be able to push 11 watts at 250 plate volts and 17 watts at 300 plate volts.

Fred
 
I think the extra gain is being limited by the feedback - try reducing the first stage cathode resistors to 4700 and increasing the feedback resistors to 10K (Reduce the 150k input resistors to 10K while you're at it). Should give another 7 dB of gain or so, especially if you stick a higher gain tube in there. 6BQ7, 6AQ8, 6BK7 are other possibilities. Once you pick a tube, some fine tuning of resistor values can minimize distortion and maximize output.
 
Great thread. I have one of these I'd like to get straightened out as well.
The other thing that perplexes me on this is what effect the 12ax7a "phase splitter" on the tuner-pre has on this amps possibilities as a standalone.
 
Are you trying for more power output before clipping (in this case before rounding), or are you looking for more gain?

In push-pull configuration a pair of 6BQ5's should be able to push 11 watts at 250 plate volts and 17 watts at 300 plate volts.

Fred


More gain up front. I'll try Tom's suggestion too (hopefully this weekend). Thanks
 
I've been working on mine for a few days, with mixed success (mostly because of parts issues). I should have RCAs, speaker jacks, and power cord done this weekend, and I'll post a pic or two.
Questions:
Mine has 6FQ7s in front of the 6bq5s - is this the same gain as the 6CG7? I see lots of references to them being the same tube, but haven't had any luck finding data on the 6Fs.
Also, I'm having trouble sourcing the coupling caps - .27uF 400v - my local has .22, and I could parallel them with .047s, but I'd rather not (also, the leads are too short). Is a .25uF going to mess this up? Or should I hold out for the .27uF? Since that would require ordering over the net, are orange drops the way to go, and from whom? (I'll be searching and reading, but I like opinions too).
 
I gotta watch this one as I have a RS 177 as well. That was the exact thing I remember reading about when deciding on which amp to do first. I know the Magnavox is pretty darn easy to run but that RCA has some nice iron on it.

:lurk:
 
The 6FQ7 and 6CG7 are indeed the same except that the 6FQ7 had an internal shield between the two sections.
For a coupling cap, it’s not that critical. What’s in there was lucky to be +/- 20% when new. I wouldn’t go lower unless I had them on hand. .33 or so Orange drops are probably a fine choice…John
 
Also, I'm having trouble sourcing the coupling caps - .27uF 400v - my local has .22, and I could parallel them with .047s, but I'd rather not (also, the leads are too short). Is a .25uF going to mess this up? Or should I hold out for the .27uF? Since that would require ordering over the net, are orange drops the way to go, and from whom? (I'll be searching and reading, but I like opinions too).

I had the same question on mine, too. I ordered orange drop .25uf 600V caps from electronic radio supply (www.tubesandmore) when I was ordering the caps for the other 2 amps. YMMV, of course.
 
Thanks for the advice so far. My local shop had some .33 orange drops, but wanted $6+ each. While I'm in favor of doing business locally to keep that option, I couldn't justify that, and bought their 4 "other" film .33s and a pair of .22s with .047s I piggy backed for less than one orange drop. Maybe I'll swap out later.

So recap of coupling caps is done. That was much quicker than the last time around. Looking at the can caps will be next, though they don't get hot at all and I'm tempted to leave them alone until I get the resistor changes figured out.

I got the speaker jacks and RCA inputs done yesterday - though I'm not really happy with the speaker jacks.

It sounds very nice on my ADS L420s, the only efficient speaker I have (and I just got them back in action today). They seem to be missing something though, and I'm not sure if that's the power or frequency response being odd.
 
I think the extra gain is being limited by the feedback - try reducing the first stage cathode resistors to 4700 and increasing the feedback resistors to 10K (Reduce the 150k input resistors to 10K while you're at it). Should give another 7 dB of gain or so, especially if you stick a higher gain tube in there. 6BQ7, 6AQ8, 6BK7 are other possibilities. Once you pick a tube, some fine tuning of resistor values can minimize distortion and maximize output.

Hey Tom -
Any chance you could ID these resistors by number on the schematic?
 
Thanks for the advice so far. My local shop had some .33 orange drops, but wanted $6+ each. While I'm in favor of doing business locally to keep that option, I couldn't justify that, and bought their 4 "other" film .33s and a pair of .22s with .047s I piggy backed for less than one orange drop. Maybe I'll swap out later.

So recap of coupling caps is done. That was much quicker than the last time around. Looking at the can caps will be next, though they don't get hot at all and I'm tempted to leave them alone until I get the resistor changes figured out.

I got the speaker jacks and RCA inputs done yesterday - though I'm not really happy with the speaker jacks.

It sounds very nice on my ADS L420s, the only efficient speaker I have (and I just got them back in action today). They seem to be missing something though, and I'm not sure if that's the power or frequency response being odd.


Does it seem to get "loud" enough with a line level input?
 
I haven't tried line level straight into it, I've been using my Hafler DH-101 or a Marantz 2275 pre-outs.
So does it get loud enough? It's listenable at about 12:00 on the volume from either preamp, and sounds full and good at close to flat out. So I think it would benefit from more gain - and even just a little will help.
 
I swapped out the first stage cathode resistor and feedback resistor as suggested by Tom above. That just about doubled the gain, I think - it seems 3-4 dB louder. I did one channel, compared, and then the other. The input resistor seems to be part of a single 3-part resistor that frightens me so that will be later.

Other than a balance issue, it sounds really, really good. (I'm a tube rookie, so take that for what it's worth). One channel is slightly weaker than the other right now. I'm sure that could be attributed to weak tubes or maybe power supply capacitors which have not been replaced yet.

For the record, mine is a RS-177J, with 2 x 6FQ7s in front of 4 x 6BQ5s. I've been working off of the RS-177H schematic, which has matched up exactly on every part I've looked at. That schematic is at page 3 here:
http://techpreservation.dyndns.org/schematics/64/64_RCA_RS-177H_134.djvu

I'll try swapping in some 6AQ8s when I have a chance to locate a reasonably priced pair on e-bay.
 
Nobody wants to give us a headsup on how to add a tube to these? :no: Come on fellers, it'll get you some good Karma for just a little typing. :yes:
 
After listening to this amp for a couple hours (after solving my weak channel problem) on a source with more signal than my i-pod, I feel pretty confident there's no need to add any tubes.
Running a CD player through various preamps, and once from the CDPs headphone jack, I know I can exceed the volume I'd want any of my speakers playing. The stereo imaging (channel separation) is dead on. I wouldn't make any changes to it.
I think making the resistor changes will put this amp into use as-is, though I'll still look to (1) reduce the input resistor and (2) try a higher gain power tube that will work.
 
That's a tough amp to start out with because it needs a lot of input to get any output. And even then, I think it sounds a little weak. I think it was designed to run a small console with small speakers.
I think it needs a little more work than simply adding a pre driver stage or decreasing feedback to get more volume. The transformers are what you are designing the circuit around and you must make sure it remains stable and doesn't oscillate. I think it requires a redesign from the ground up and a potential money pit because the output transformers may not be up to the task. I gave mine away, and the guy I gave it to gave up on it too because it never sounded strong. He put a head amp in front of it and it still sounded weak. I just don't think the output transformers are designed for big speakers.
I don't think that RCA amp does the el84 tube justice and think you might enjoy another amp entirely. I found a Fisher 481-A chassis that blew that RCA out of the water. When I gave my RCA away I did so without hesitation and warned my friend not to get too into it.

Just my 2 cents. Good luck.
 
First this thread is over 7 years old yur responding to. :)

But..
Actually it is a pretty decent starting point for a budget amp.
Just the gain on the tubes is to low to run any feedback there.

I've done a couple, using 40-60 gain tubes like 6bk7/12at7 an
tweaking the circuit for decent feedback and it runs well there.
 
I think the extra gain is being limited by the feedback - try reducing the first stage cathode resistors to 4700 and increasing the feedback resistors to 10K (Reduce the 150k input resistors to 10K while you're at it). Should give another 7 dB of gain or so, especially if you stick a higher gain tube in there. 6BQ7, 6AQ8, 6BK7 are other possibilities. Once you pick a tube, some fine tuning of resistor values can minimize distortion and maximize output.

I did the mods above.

It is quite easy, you only need 10k resistors.

* Just added one paralell with the cathode bias resistor

* one paralell with the input resistor (leg 1 and 2 of the component)

in these cases the R(total) is near the suggested values

Next I disconnected the feedback resistor at the terminal lug and added the 10k

Also, I put NOS 6BQ7A tubes in place of the 6FQ7

I am using the amp with a Hafler DH-101 preamp and JBL LXE770 speakers. The volume half-way and the system is pretty loud and nice.

My avatar is actually this amp!
 
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