Traynor YBA-1A

[gui_tarzan]

Thought I'd start a new thread so I don't continue to hijack a different thread. I have a 1971 YBA-1A head and while replacing filter caps I noticed a few oddities.

No grid stoppers and only one 470 ohm screen resistor.
Pins 1 on the 6CA7s are disconnected, the schematic shows them connected to pins 8.
1 ohm 10W resistors on pins 8 to the chassis, none shown on the schematic
No caps on the bias circuit, they are shown on the schematic.
No .1/600 caps on the PI plates going to the power tubes even though the schematic shows them.

G-one suggested putting separate 1K resistors on the grids. I only have 470 so I'm trying those first (ala Fender), and 5.6k on the stoppers, which I did both of. I also added 8/150 caps on the bias circuit and jumped pins 1 to pins 8.

My plan is to add the .1 caps on the PI grids where they're missing. I included the schematic with the changes I made for your review.

[audiopete]

The reason 1K was recommended for the screens, not grids, is because 470 ohm doesn't protect the screens well enough. Fender didn't know everything. You should order 1K/5W and put them in when you can. Like these - https://tubedepot.com/products/5-wat...-grid-resistor

No coupling caps to the power tubes? PI plate voltage on the power tube grid 1? I'd imagine that wouldn't be good.

Do you have any gut shots? Someone put the 1 ohms in to be able to measure cathode current with a voltmeter. I prefer a bias probe that goes between tube and socket.

Looks like this amp has been through the ringer a little.

[gui_tarzan]

I did some digging after I posted that and found some 1K/5W resistors so I'll put those in in place of the 470s I put in. I also changed the 10k PCB bias pot that was just barely hanging on with a broken lead with a 10K standard bias pot and put it in a hole in the chassis someone else drilled at some point in the past. I suspect they had the same idea but never got around to doing it.

I'll also put some F&T caps in place of the IC caps you see in the pic the next time I get some. The two terminal strips I added will also be taken back out, I thought I would need them with the placement of the new caps but it turns out I don't. I used some 40/40/450 cans from CE to replace the original 40/40 cans so I'm not sacrificing real estate inside the chassis.

The 1 ohm resistors won't hurt anything so I'll probably just leave them there for now, but I have a pair of bias testers that I'll probably use instead. I still need the ability to check the plate and cathode current so I'll have it open to do that anyway.

[g1]

Something is very wrong here. Either it didn't work and was very butchered, or you need to take a real hard look at the wiring.
Without coupling caps from the PI plates you would have major red-plating of the power tubes. With no bias caps you would have big time hum. I see a couple bias caps in the picture, was that pic after you added them? Did you wire up pin 1 of output tubes to pin 8 ?

[gui_tarzan]

I agree, it was missing parts and was in general a mess. Yes, I added the bias caps and jumpered pins 1 & 8 on the output tubes. I also just put the .1/600 caps in. I suspect that's why I was getting the red plating now. Go figure.

I went through the entire circuit after realizing the bias was messed up, that's how I discovered the missing parts.

[nevetslab]

In looking at your 'gut shot', I see what looks like the two Electrolytic caps from the Bias circuit....just to the left of the rectifier diodes...caps black in color. And, it looks like there used to be coupling caps between the LTPI stage and the grids of the output stage. Right by the middle hex nut on the circuit board, there's a cap that resembles a dip tantalum, but it's really a 470pF Ceramic cap. It's across the plates of your LTPI drivers. The other side of the board where it looks like your grid wires connect to your power tubes, those caps are, as you said, missing. I'm not seeing in the photo how your circuit connects from the driver stage to the power tubes.

Oh...I just re-read your post, and see you DID install those bias filter caps prior to the photo posted. My mistake.

With Pin 1 disconnected from Pin 8, I suspect this had been set up for 7027 Power Tubes. Internally, Pin 4 (screens) and Pin 1 are tied together on that tube. For EL34/6CA7, you'll want that re-connected, as there's no internal connection between the cathode and the suppressor grid, which is Pin 1 on the EL34/6CA7.

What tubes are installed in the amp?

I'd also recommend removing the Ground Reverse Cap, disconnecting that switch associated with it (the Death Switch), and changing the 2-wire power cord to a 3-wire Grounded cord. Never was a big fan of getting lit up with fingers on the strings and lip touching the mic at a different ground potential.

[gui_tarzan]

I already removed the death cap and polarity switch wires when I put a three-wire power cord on it. If memory serves I got this at a yard sale some twenty years ago without any tubes in it. Once I found the right schematic I purchased the correct 6CA7 tubes and have now verified that they are wired correctly.

[g1]

Ok, sorry, for some reason I had thought you had a working unit so it didn't make sense. Sounds like you are making good progress. Good luck and keep us posted.
(P.S. Those suggestions in the other thread about the grid stoppers and screen resistors were from Gtr_tech not from me.)

[gui_tarzan]

Oh, sorry.

At any rate unless you guys see anything I missed I think I'm close to checking voltages.

[Enzo]

I came into this late...

Without coupling caps from the PI plates you get red plating? I guess that is how one could read the description. MY first thought of missing coupling caps was how would th signal get there then? Were they missing or wired across or some other value? Otherwise I'd have to agree that PI plate voltage on power tube grids would cause havoc.

EL34/6CA7 won;t work right with a disconnected pin 1, but some YBA-1 amps used 7027, and those would be fine with no pin 1. As would 6L6. That might explain it. OOps, someone said that already.

I think the single screen grid resistor is explained by considering that the heavy screen currents we need to limit only occur on one side at a time of the push pull, so the single resistor can serve both grids. I am not necessarily defending it, just explaining it. And remember, Fender was working with sturdy 6L6s, while EL34s have much more fragile screen grids.

What is wrong with 1 ohm cathode resistors? That is exactly what is inside a bias probe socket adaptor. So either way you are measuring voltage drop across a 1 ohm resistor. In fact, you can leave it in anyway, as 1 ohm will have zero effect on the operation of the amp and will not affect the operation of a bias probe adaptor either.

[gui_tarzan]

Yeah, there are a bazillion different schematics for the YBA series, it took hours to find the right one. I'm excited to hear what it sounds like with the 6CA7s, even though my thought is a stout set of 6L6GCs would be better for bass which is what I play mostly. I could be wrong, I'll find out soon.

To answer your question, I was checking voltages with just the output tubes and I could see the plates on one start to turn red so I shut it off and started looking it over. My mistake, I should have looked closer before I got to that point. It didn't occur to me that the grid stoppers not being there could be an issue because they're not on the schematic, then I looked deeper and saw the other problems. That was a rookie mistake I never should have made. Lesson learned.

[Enzo]

Ah, the first thing to do is check the voltages at the output tube sockets WITHOUT the output tubes. That way you can verify there is B+ at pins 3 and 4 and good bias at pins 5.

[gui_tarzan]

Yeah I know, I feel stupid now for doing that prior to checking it out really well. Like I said, lesson learned. I was just excited to see how good this sounded since I haven't touched it since I bought it so many years ago. I've worked on so many amps lately for other people that just needed the basics I forgot to check my own stuff out thoroughly first.

[Mick Bailey]

I think the single screen grid resistor is explained by considering that the heavy screen currents we need to limit only occur on one side at a time of the push pull, so the single resistor can serve both grids.

In the Mullard EL34 spec (1962) they give the connection as "Screen-grid resistor common to both valves" for P-P operation.

I saw an old paper outlining the benefits of this approach but haven't been able to relocate it. One view is if a fusible resistor is used, then a single shorted screen shuts down all the tubes. I like the idea of low-wattage fusible resistors for screens anyhow. A 5W WW resistor can pull a very large current before it lets go. Especially if there's no HT fuse. The single shared resistor is a feature of some Traynors, for whatever reason.

I've read of sonic benefits of the single shared resistor - does anyone have more information?

[pdf64]

See gingertube's take on the shared screen grid resistor The Amp Garage :: View topic - Screen resistors - please weigh in