SOLVED Tube Identification based on circuit

[PunchySunshine]

Hello you Wonderful Wizards!
I'm restoring a small homemade tube amp and I've run into a road block. I've replaced all the filter caps. The only caps left are the two "tone" caps but I'm about ready to replace those as well JIC. They measure at 8nf and their label says they should 5nf. The sound I'm getting out of the amp is weak. The filaments are all getting voltage and lit. I made a diagram in hopes that someone here can get me going again. The rectifier is a 6x5GT. That is the only tube I can properly identify. I think the power tube is a 6v6 but I can't tell and I don't have a clue what the pre tube is other than is ISN"T a 6SJ7 like I was hoping. I know it's a long shot but maybe @RetiredUnit1 or @Chuck D. Bones or @owlexifry has some clue for me to get me going on this again. I just feel so very close but yet so far!
View attachment 63009

 

[Erik S]

I have a Sencore TC162 tube tester, it's got a cool feature that can tell you how much life a tube still has. And it's solid state, and tests transistors as well. It's a LOT easier than putting it together and THEN having to look for the faulty component.

I've been watching one of these on Craigslist near me. Just realized you referenced the same model - that was enough to push me over the edge. Just sent off a message.

Sencore TC-162 Tube Tester - electronics - by owner - sale - craigslist

This is a nice Sencore TC-162 tube tester that is in very good working condition. It has been serviced and has a new tube setup book that comes with it. Asking $140

nh.craigslist.org

 

[PunchySunshine]

Ok, I've ordered the five resistors in both Carbon and Metal. I've also ordered a new rectifier but I'm skeptical for this to fix anything as I've got seemingly solid B+ on the 6v6.

What I'm having trouble wrapping my brain around is WHERE is the cathode power coming from for both the pre and power tubes?

I've got super low voltage on the pre tube and I suspect this to be the problem with low output, but I cannot for the life of me figure out where the cathodes supposed to be getting their voltage on the pre and power.

 

[RetiredUnit1]

Ok, I've ordered the five resistors in both Carbon and Metal. I've also ordered a new rectifier but I'm skeptical for this to fix anything as I've got seemingly solid B+ on the 6v6.

What I'm having trouble wrapping my brain around is WHERE is the cathode power coming from for both the pre and power tubes?

I've got super low voltage on the pre tube and I suspect this to be the problem with low output, but I cannot for the life of me figure out where the cathodes supposed to be getting their voltage on the pre and power.

The Cathode is the ground for the tube, and controls the frequency response. This is a quote I copied a while ago to help me keep things straight in my mind. Pic at the bottom is a layout I did of the Fender Champ I'm *going to be* making, eventually. I have so many projects in the queue, lol. The brown wire on the preamp tube leads from the cathode to the R/C to ground, the thinner black wire on the power tube runs from the cathode to the R/C with a five watt resistor to the ground. *Edited: Note I've separated the power tube ground and preamp ground to eliminate the possibility of a ground loop.

As you can see it's a great example of a simple design like the one you're working on, but has a circuit board instead of point to point wiring. It's very easy to make circuit boards. I literally print this on paper, glue it to epoxy board that I saw with a wet tile saw, drill the holes and 'swag' in the turrets or eyelets depending on which I'm using. I've added a few things to the standard Champ layout which is why I called it Champ Deux (Champ 2 in French, I'm not French but I like the word Deux and Champ 2 was already taken by Fender!).

You have a negative feedback loop between the grid and cathode. No cap = more feedback and lower gain. Bigger cap = less feedback and higher gain. Smaller caps only boost highs. (like a presence control) The bigger the cap gets, the flatter the response, until you hit somewhere around 22uf and the boost is flat through the audible range.?? If you hit a big bassy chord, that cap discharges, and you temporarily lose a bit of low end. Start making that cap larger and larger, the harder it is to discharge, the steadier the bias supply, and the steadier the bass. Useful to some, boomy muddy mess to others.

 

[PunchySunshine]

The Cathode is the ground for the tube, and controls the frequency response. This is a quote I copied a while ago to help me keep things straight in my mind. Pic at the bottom is a layout I did of the Fender Champ I'm *going to be* making, eventually. I have so many projects in the queue, lol. The brown wire on the preamp tube leads from the cathode to the R/C to ground, the thinner black wire on the power tube runs from the cathode to the R/C with a five watt resistor to the ground. *Edited: Note I've separated the power tube ground and preamp ground to eliminate the possibility of a ground loop.

As you can see it's a great example of a simple design like the one you're working on, but has a circuit board instead of point to point wiring. It's very easy to make circuit boards. I literally print this on paper, glue it to epoxy board that I saw with a wet tile saw, drill the holes and 'swag' in the turrets or eyelets depending on which I'm using. I've added a few things to the standard Champ layout which is why I called it Champ Deux (Champ 2 in French, I'm not French but I like the word Deux and Champ 2 was already taken by Fender!).

You have a negative feedback loop between the grid and cathode. No cap = more feedback and lower gain. Bigger cap = less feedback and higher gain. Smaller caps only boost highs. (like a presence control) The bigger the cap gets, the flatter the response, until you hit somewhere around 22uf and the boost is flat through the audible range.?? If you hit a big bassy chord, that cap discharges, and you temporarily lose a bit of low end. Start making that cap larger and larger, the harder it is to discharge, the steadier the bias supply, and the steadier the bass. Useful to some, boomy muddy mess to others.

View attachment 63008

Ok, that makes sense. Cathode is the ground for the tube.
So then where is the plate getting it's dc from in the pre tube (V3) in my circuit?
V3 plate voltage is 3vdc when it's off. It jumps up to 250vdc on warming up. Then settles at around 48vdc idle. If I put a 1khz tone at the input I get down to 16vdc at the plate. This negative feedback loop has me befuddled.

 

[RetiredUnit1]

Negative feedback is the name of the process where an inverse wave form is fed back into the signal. Why?

If you take the same wave form, and run it through two different speakers pointed at each other, and one of the speakers has the hookup to the amp reversed, there will be no sound! The two wave forms will cancel each other out. (This is why it's really important when hooking up a stereo to be sure red goes to red and black goes to black.)

This is how they make quieter engines and silent helicopters. Mics and speakers strategically placed cancels out the sound.

In the tube amp, just the right amount can keep it from breaking up. I.E. a clean amp instead of a roaring monster. So adjusting this can have a major impact on the type of amp that is created.

In the tube, the heater causes electrons to float freely inside the vacuum. The signal is used as a magnet to attract those electrons, they go wizzing past and slam into the plate in the exact frequency as the signal thereby amplifying the signal.

So the connection in a tube is LITERALLY through space. The electrons just go where they are attracted.

I always get the names confused, I'm old. And I read the book 20 years ago, lol. But in a nutshell this is the science behind tube amplification.

 

[PunchySunshine]

Negative feedback is the name of the process where an inverse wave form is fed back into the signal. Why?

If you take the same wave form, and run it through two different speakers pointed at each other, and one of the speakers has the hookup to the amp reversed, there will be no sound! The two wave forms will cancel each other out. (This is why it's really important when hooking up a stereo to be sure red goes to red and black goes to black.)

This is how they make quieter engines and silent helicopters. Mics and speakers strategically placed cancels out the sound.

In the tube amp, just the right amount can keep it from breaking up. I.E. a clean amp instead of a roaring monster. So adjusting this can have a major impact on the type of amp that is created.

In the tube, the heater causes electrons to float freely inside the vacuum. The signal is used as a magnet to attract those electrons, they go wizzing past and slam into the plate in the exact frequency as the signal thereby amplifying the signal.

So the connection in a tube is LITERALLY through space. The electrons just go where they are attracted.

I always get the names confused, I'm old. And I read the book 20 years ago, lol. But in a nutshell this is the science behind tube amplification.

I totally get phase alignment and how it applies to NFB.

I don't see how / where V3 is getting it's DC?

I sure appreciate you taking time and breaking this down for me amigo! I feel like a teenager with my first motorcycle torn apart in the backyard and from over the fence, the neighbor enlightens me to the concept of "top dead center".

 

[RetiredUnit1]

I totally get phase alignment and how it applies to NFB.

I don't see how / where V3 is getting it's DC?

I sure appreciate you taking time and breaking this down for me amigo! I feel like a teenager with my first motorcycle torn apart in the backyard and from over the fence, the neighbor enlightens me to the concept of "top dead center".

I see what you mean. I am unfamiliar with the preamp tube, but there are several designs I've seen where the DC in the tube is chained to the next tube. In this case I'm thinking the primaries for that weird second transformer (never seen anything like this) connect through the tube to supply the 43.5vdc to V3. Kinda the only possibility.....

edited: and to have DC connect through a 500k resistor to AC heater??? w....t....f....?

 

[PunchySunshine]

 

[RetiredUnit1]

View attachment 63076

Well this is much easier to understand. I was totally confused by the PTP wiring.... There's actually a forum group of people that prefer this type of wiring over circuit boards. I was amazed by that. The main reason they touted was "it takes too long to make a circuit board". Umm... I can make one in an hour from printing out the template to being finished....

 

[PunchySunshine]

Alright, calling it solved. The preamp tube can be either a 6C5 or a 6J5. Just needs to be a single triode. That being said, not much gain at all. My buddy who gave me the project believes this was an external speaker for a projector on a school AV cart back in the day. It originally had an RCA input. This got me thinking to try it after the pedalboard. It's still soft, but UBER clean:

New video by Dylan Saliba

photos.app.goo.gl

Now I'm gonna try to put in a 12ax7 and give her the onion!