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Discussion Starter #1
I repair amps on a regular basis. Anyone have experience with a YCV20 with above normal hum? Perhaps it's just the design, but it has noticeably louder buzz than other tube amps, but is reminiscent of a Fender Pro Jr in this respect.

any suggestions are welcome. Otherwise, I'll update this thread as I continue to troubleshoot. I've already done many of the basic checks and tests.
 

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I have the same amp in wine red and did not get any hum at all until my tubes started going. One of the reasons I bought this amp was because it was so quiet - apparently something to do with a DC filament. I'm sure you have way more knowledge about amps than I do but I thought I'd pass on my experience with it.
 

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Gunny said:
I repair amps on a regular basis. Anyone have experience with a YCV20 with above normal hum? Perhaps it's just the design, but it has noticeably louder buzz than other tube amps, but is reminiscent of a Fender Pro Jr in this respect.

Are you sure it's not normal to the design? Silly question I know but if they all have a similar amount of hum you'll be wasting a lot of time.

Assuming that something indeed has gone wrong to cause the problem then I would suspect something like a heater to cathode short in one of the tubes. If you don't have a tube tester then you could replace tubes one by one and see if you can locate a bad one. Cathode/heater shorts are sneaky! Depending on the tube they may not cause a big failure and pop a fuse. They can simply defeat all the filament hum reduction parts of the circuit.

Looking at the schematic I would think this amp would be quieter than most! The output tubes are fed from one 6v winding with the usual pair of 100 ohm resistors making a hum centretap to ground. The preamp string is actually fed from IC voltage regulators. A 7815 or a 7915 provides super clean 15vdc that is fed to the 12AX7s through a small resistor to give 12vdc. There shouldn't be any hum to speak of from the filament supplies.

That would leave only the signal paths. You could have a poor ground at the input jack. It may have a bit of oxidation on the contacts. The effects in/out jacks could also need cleaning. The jack itself may have been bent inside by some player being a bit clumsy. Or physical shock may have broken the connection to the pcb.

Given the limits of the info provided, that's all I can think of for a first pass. I'd be putting my 'scope at various points to look for any buzz that shouldn't be there.
 

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Discussion Starter #4
Thanks Wild Bill. I tried a swap of the input jack with no improvement. It uses unshielded wires to the PCB so I installed a shielded (at one end only) coax for that connection.
The owner put new JJs in all tube positions. I swapped out the 3 preamp tubes with no change then reinstalled the originals. If you pull a single output tube, the noise vanishes. With 2 output tubes, the noise (hum) is back.

I'm going to check the bias in more detail. It's a cathode bias arrangement.

I suspect these are running full out (by design choice). Perhaps cutting back to about 8 Watts plate dissipation per tube will help the problem. I'll report back on how this turns out.
 

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Aha! Ahahaha!

Gunny said:
Thanks Wild Bill. I tried a swap of the input jack with no improvement. It uses unshielded wires to the PCB so I installed a shielded (at one end only) coax for that connection.
The owner put new JJs in all tube positions. I swapped out the 3 preamp tubes with no change then reinstalled the originals. If you pull a single output tube, the noise vanishes. With 2 output tubes, the noise (hum) is back.

I'm going to check the bias in more detail. It's a cathode bias arrangement.

I suspect these are running full out (by design choice). Perhaps cutting back to about 8 Watts plate dissipation per tube will help the problem. I'll report back on how this turns out.
Actually, it looks to me like the tubes are not running maxed out at all! Usually mfgs of guitar amps run 6BQ5/EL84s pushing 350 volts on the plates. Here Traynor is running only 270vdc. A pair of these tubes might be asked for 30 watts in a Peavey. Yet this amp claims 15 min, in its spec sheet.

It looks like this amp runs the tubes in class A, or close to it! Great for bluesy vintage tone! The power output and the plate voltage couldn't really be much else. In class A the tubes are run at the plate dissipation figure. This means lower power and plate voltage levels or else the tubes will burn out. The tube(s) is always fully conducting so it has no "off" time during its duty cycle. With the more usual class AB1 one tube will be shut off nearly half the time so it has a moment to cool. You can run them at hotter levels, hence you get 30 watts outta that Peavey.

There's something about class A amps that many designers don't know or forgot. Normally you don't need much filter cap mfds at the plate node with class A. The current in the output tranny winding is balanced on both sides throughout the entire duty cycle and that in itself cancels out tons of hum.

Did you read the user's manual? It trumpets how the amp is "auto-biased" and you don't need matched tubes. This isn't totally true at all...

It's true that cathode bias tends to automatically correct itself. The bias comes from the cathode current running through a resistor making a voltage drop that raises the cathode above the grid, in effect making the grid voltage negative like with a fixed bias circuit. If the tubes are hotter the current will be higher so the bias voltage generated will also be increased to throttle back the tube(s). With cooler tubes you get less bias and the circuit will run them hotter.

In the YCV20 the tubes have a common cathode resistor. What happens if the output tubes are seriously mis-matched?

First off, there's no individual tube auto-correction to be had. The cathode resistor sees the combined current of both tubes and gives an average bias voltage. IF each tube had its own cathode resistor (the ohmic value would be twice that of a common resistor) then each tube would be auto-biased correctly for itself but that's not the case here.

Now, we likely have a big unbalance in the current flows in each half of the output tranny winding. So we don't get that hum cancellation we expected. What's more, the value of the filter cap is 22 mfd. OK if things are kosher but with a big mismatch in tubes it's kinda small! It won't filter much of that unexpected hum ripple at all.

When you said the hum disappeared after pulling just one output tube that was the clue! Try a matched set!

So despite the claims in the user's manual I would ALWAYS use close-matched output tubes! What's more, I'd bump up that 1st filter cap value to at least 47 mfd, on general principles in case the tubes drift apart over the months.

Another area to look at would be adding a bypass cap of maybe 47 mfd/25v or bigger across the cathode resistor. This is another clue that the amp is designed for class A. You don't need to bypass the cathode resistor in this mode. However, if things are unbalanced and AFU then a bypass cap would help "big time"!

Took me a lot of cussin' over the years to figure out that you can't always assume that the amp was perfect in the 1st place!:mad:
 

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I have the same amp and it is super quiet.

Traynor has an outstanding warrenty and could help you out at the very least they may have some info on what the problem could be.

All the YCV amp I tried were quiet so it may not be a design thing, not that i'm a tech.. just heard a few.
Good luck
Bev
 

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I took a YCV20 home for trial a couple of years ago and was impressed with the quality and sound but I was bothered by the fact that they put the pre-amp tubes inside the chassis. At the time I didn't think it was such a good idea because it would provide for much heat inside the chassis itself. I know they provided some minimal vents but I wasn't sure if it was adequate enough. I figured I'd wait for a few years to see if this would lead to any problems but so far I haven't heard of such. Part of me though felt with a 5 year transferable warranty the designers, most likely did their homework on this design.
 

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Thanks, Bevo and bRian!

There's two reports that say the amp is normally super quiet. Given the circuitry, it should be.

Yours isn't! What's different about yours? Again, the likely answer is badly mismatched output tubes. If you don't have a tube tester or a known good pair you can try measuring the voltage drop across each screen resistor. If the tubes are running similar currents the screen currents will also be similar. Knowing the voltage drop and the resistor value means its just Ohm's Law to figure the screen current. If they're out of whack you'll know really quickly.

There are some other possibilities but they run to much more trouble and money. let's not go there unless we have to...:eek:
 

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I am not a tech either but I had a problem back a while ago with my ampeg, even with the volume down there was this annoying hum present unless I completely turned down the bass control to 0. One day after getting PO'ed about the situation I happen to find a pair of 12ax7's that were up on the shelf. I replaced the 2 12ax7 preamp/inverter tubes and bingo, the noise was greatly reduced, now quite liveable.
 

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Is the amp a new one under warranty? If so just call Yorkville and get it replaced. They have a great warranty.

I have played a lot of the YCV amps and they are among the quietest tube amps I have played. So I would have to think something is wrong with it. With the warranty those amps have, I'd replace it over putting time/money into fixing it.
 

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Discussion Starter #11
update

Thanks for all the input everyone. I listened to my own 2 tube amps and they are dead quiet also.
As Wild Bill predicted, bias had no effect. I already had tried a filter cap across the cathode resistor....tried it again...still no difference.
Measuring screen grid voltages, I get identical readings of 270 Vdc. I could do a transformer shunt current measurement, but the HV connection is a push-on and you can't easily get to it with a test lead. I may have to pull it off partially to try that.
I was so keen to get at this amp I forgot to ask if it was still under warranty. I guess I'll have to try another set of power tubes (matched) to see if that fixes it. Otherwise, I'm all out of ideas.
I had previously bridged power supply filter caps. I know that's not a perfect way to test, but OK in a pinch. No change in symptoms.
 

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Discussion Starter #12
Dropped a new set of (for sure) matched output tubes in the amp. No improvement : (
 
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