Almost there...Just need to smooth out the rough

[rcrmck]

The issue: It sounds great until I push it, and instead of nice smooth overdrive and sweet musical distorsion, it sounds more like a rusty old buzz-saw missing a couple teeth. Could it be as simple as old capacitors? If so, which ones?

The low-gain channel is nice and shimmery-tube cool, especially with my acoustic, all the way to full volume, but it doesn't get very loud. I can see using that channel regularly. In the high-gain channel, it starts overdriving at about 2/10ths volume, and is very rough and buzzy-poppy jumpy, not smooth at all. It will get extremely loud, and I start getting feedback in the closed room at about 6/10ths volume, and by that time, the distorsion is not as jumpy but still very buzzy and not musical. I switched the 6eu7 tubes and nothing changed.


Background:
Okay, so I got the Telepage amp from my father. I never could find a schematic or other info on it, but it looks simple so I made some minor changes. It was working as a pager in an old warehouse up to about 10 years ago, and then sat in storage until now.

It has 2 6eu7 preamp tubes a 6sn7, a pair of 7868 power tubes and a 7199. It has a phono input that goes to one side of a 6eu7, that I will use for a low gain channel, and a mic input that goes to both sides of the other 6eu7, that I will use for a high gain channel.

Thanks

[loudthud]

Not enough info to go on. Did you replace any of the filter capacitors? I think it's safe to assume the 7199 is the phase inverter driving the 7868s. That's probably good for about 30 watts like the circuit in the back of the RCA tube manuals. The 6EU7 is pretty close to a 12AX7 but with a different pin out. What is the 6SN7 for?

[rcrmck]

No, I have not replaced any capacitors at all.
I don't have a schematic, and I'm pretty new at this stuff, so I just took it for granted everything downstream of the 6eu7's was good, since I didn't change anything. I'm actually confused some, because I thought the 6sn7 was the splitter. I thought that the 7199 was a rectifier because I was unfamiliar with that tube and there are no other tubes present to do that job, but I see it would probably be a splitter. So I don't know about the 6 sn7 now. What kind of information can I look for that will help in getting this problem nailed-down?

I've heard a lot of SS amps that sound horrible when they start clipping, but have you ever heard a tube amp that sounded bad when it started to overdrive?

Thank you for the help.

[loudthud]

The 7199 is not a rectifier. It contains a pentode and a triode. Can you post pictures of the amp inside and out?

Tube amps with old worn out dried up filter caps can sound pretty bad. This amp probably has a multi section can cap, sometimes called FP type. Usually the ratings will be printed on the side. You will probably have to improvise some way to mount the new caps.

Finding a good pair of 7868s won't be inexpensive. You may have to convert the amp to 7591s which will require new tube sockets or some other octal tube. EL84s probably won't get the job done unless you can squeeze four of them in there.

[rcrmck]

Yeah, I kinda thought I would be replacing those capacitors eventually anyway.

I'll take some close-up pictures when I get home.

Thanks for your help.

[überfuzz]

The issue: It sounds great until I push it, ...

In the high-gain channel, it starts overdriving at about 2/10ths volume, and is very rough and buzzy-poppy jumpy, not smooth at all. It will get extremely loud, and I start getting feedback in the closed room at about 6/10ths volume, and by that time, the distorsion is not as jumpy but still very buzzy and not musical. I switched the 6eu7 tubes and nothing changed.

Hmm, 2 on that pot isn't really pushing it is it..? By the way. Are you sure something is busted? I've had a couple fender amplifiers, hand wired, that where goofing as a result of how the wires were situated in the chassis. You can get capacitance between chassis and wires for example, between wires also.

[Rhodesplyr]

I've heard a lot of SS amps that sound horrible when they start clipping, but have you ever heard a tube amp that sounded bad when it started to overdrive?

Yes :-)

It's tricky not being able to see the circuit, but there is a situation where a cathode-biased Class AB1 output stage has large cathode bypass capacitors to extend the range of "clean" power. This creates a large time-constant for these capacitors in relation to the cathode resistor(s), and what can happen is that being driven into distortion makes it sound really nasty because the time required for the output stage to recover is so long. This is especially the case at low frequencies. If it is cathode biased with bypass capacitors, you might play with making them smaller and/or you might end up making signal path coupling capacitors smaller to limit the bandwidth.

Also, triodes and pentodes produce different distortion harmonics. With triodes, you get even-order harmonics that are more euphonic; with pentodes, you get lot of odd order harmonics that have more of an edge to them. Perhaps you're overdriving the pentode section of the 7199.

In general, some amps that were designed only to run clean can sound pretty harsh when overdriven.

For loudthud:

Good news! Electro-Harmonix is now producing a 7868, so you don't have to rewire for 7591s.

[Steve Conner]

Yes, if an amp wasn't designed to work with guitar, there's no guarantee that it'll sound good when you overdrive it with a guitar. Some tube PA amps do, others don't.

The guidelines given by Rhodesplyr are about right, you might need to make a bunch of capacitors smaller.

[rcrmck]

Okay Got a couple pics

I attached a couple pictures. Hopefully something stans out.

[tboy]

I don't see a 6SN7 in the picture, but I do see a 6CA4 - that's the rectifier.

Contact bias was often used in the early preamp stages of tube PA amps, and overdriving a contact-biased stage very far does tend to sound like crap. A contact-biased stage will have it's cathode connected directly to ground (no cathode resistor), and a very high value grid resistor, like 5 megohms or more. If you have any stages like that in your amp, converting them to cathode bias should improve things.

I would highly recommend drawing a schematic that shows what you've done so far; without that, we're all just guessing.

[loudthud]

For loudthud:

Good news! Electro-Harmonix is now producing a 7868, so you don't have to rewire for 7591s.

I thought I remembered seeing that somewhere but I looked at AES and all I could find was NOS for something like $54 each. Ouch! I wonder if they have the high transconductance of a real 7868 or are some re-pinned Russian tube like the 7591s.

[rcrmck]

I don't see a 6SN7 in the picture, but I do see a 6CA4 - that's the rectifier.

Oh man.. I'm sorry. I did write down the wrong tube on my notes. I guess that at least makes more sense. I'll try sketching out the preamp section as best as I can, and look for that.

[Rhodesplyr]

Speaking of tubes, have you verified that they're all in good working condition?

I had to sketch out the circuit of an amp I was repairing that had reverb and tremolo. This one is pretty simple, so I don't think it would really be hard at all. All you need to do is make sure you know which tube pin is which. I can already see that it uses the same type of phase inverter/driver as the Dynaco ST-70.

Also, is it just the photo, or is the phenolic mounting wafer of the power supply capacitor can discolored? That could mean that that capacitor has been leaking, but I can't tell for sure from a photo. It could just be heat discoloration. While it's convenient, the practice of mounting power dropping resistors on can capacitor terminals usually shortens the life of the capacitor by heating it from underneath.

It is a fixed bias amp--I can see the negative bias supply. There's a tan-colored capacitor in the upper left of the photo that's a wax-dipped paper capacitor. If it has any DC voltage across it, it's likely leaking, and they tend to change value and increase in ESR rather radically over time. I automatically replace any of those I find. The Black Cats generally survive fairly well. I've never seen caps exactly like the big .1uF/600V ones that couple to the 7868s.

[rcrmck]

Yeah, I noticed that white discoloration on the cardboard under the can capacitor too. The tan capacitor is waxy paper, and so are the orange ones. Messing around I just noticed the 2 big black ones (beaver) are dry cardboard, but the cardboard shell will spin around the axis real loose like. I guess I should start by replacing all those, and go from there.
Thanks! I think that gives me some hope. My dad is almost 80 and I think he'd get a kick out that amp getting reincarnated into something useful.

[rcrmck]

Woo-Hoo!!!
I replaced all the paper/wax and can capacitors and that did it! It's still really loud, and it goes smoothly into distortion. There is a very minor hum as I bring the volume up, but nothing too bad at all. I'm pretty happy with it now. Next is some paint for the front plate and cover.

Thanks guys!!!