Traynor YBA-1a Bias issue

strangeways

Member
2016-05-16 10:25 pm
Hey all,

I'm an occasional amp builder and belong to a few other forums, but it seems Metroamp and ampgarage are non functional, so I figure I'd check here for some help.

I've got a Traynor 69 YBA-1a MKII that recently started giving me issues. Specifically, I noticed last time I was using it tubes were red plating slightly. Put it aside and figured it needed a retube. After opening it up and checking bias voltage (no power tubes in it, on standby), seems the bias supply voltage has wandered up to around -100v. After changing R33 (150k/2w) the diode following it, r32 (15k) C13 and C14 R31 and swapping out Cll and C12, no improvement.

What I'm noticing that has me baffled is voltage after R33 is -63v, which seems correct. But after the bias diode (PH204), I'm getting 133v. And after the 15k resistor, I'm getting around -105v. It would seem the diode may be the problem, but I replaced the original with a new one with no improvement. The amp has a 10k bias pot tied to the 47k resistor that apparently came in this model, since it's in the schematic inside the amp. I also replaced C18, just because I had a new cap around. I really don't want to dump a ton of money and time into recapping the entire amp, but if that's what it takes to get it going, I'd guess that's the next step unless someone here has the cure.

Thanks for any help.
d.

I've
 

strangeways

Member
2016-05-16 10:25 pm
Thanks. I realized after posting it was in the wrong forum.

In recent news, I changed the 47k bias resistor to 15k and now can get -47 volts for bias, which is what the schematic says I should read at the bias pot/Bias resistor test point. Unfortunately, still getting red plating on tubes. Can't really do any testing since I have to keep watching so I don't toast my 1 good pair of tubes. This is driving me crazy!!!!!

D.
 
Having a more negative bias voltage shuts the tube off more. It makes it colder not hotter. I had an amp that the old bias adjust pot would go open intermittently. The tube's plates really did get red on that one! You want a more negative voltage to reduce the bias current.

So it sounds like you have two issues. The first is why the power tubes aren't being throttled back with the bias voltage. The second is why you had to change the 47k to get to the proper bias voltage.

There are two schematics. Bwhich one is correct?
 
The voltage after R33 is unrectified AC. Look at the top of the preset and adjust for -46v. If it is -46 and still getting too hot with a new set of EL34s, try increasing to -50. Also check the HT on the screen grids is not too high.

There is an outside chance the DC blocking capacitors are leaky but not likely to be both of them!
 
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mozz

Member
2016-05-21 7:00 pm
Remove the output tubes, check for the neg bias voltage right on the pin 5. Be sure you have screen and plate voltages right on the pins. How is the plate voltages on the PI? Open grid block resistors? 1.5k Open bias pot, or ground. Also, take those spike diodes out off the output tube plates, they start to leak.

These amps are well worth the money to recap. Don't change the mustard caps if they measure good. I think all the power resistors were out of tolerance but the smaller film resistors were all good. My plate voltage is 575, a bit higher than original. Mine is a 1972 and it is on the bench as we speak, plexi mods and trying to get the b+ down a bit to more sane levels.
 
Hi Guys

Before you install tubes, resolder all of the connections related to the grid circuit of the output tubes and the bias supply. Then check that bias voltage appears at the output tube sockets WITHOUT TUBES.

The control grid voltage is important inasmuch as it keep the tubes reined in and not melting down. However, the critical parameter is cathode current.

Insert a 1R-1W metal-oxide resistor between pins 1&8 and ground. This will let you make a ground-reference voltage measurement that corresponds to the current through the tube as 1mV=1mA. In the YBA1-1A plate and screen voltage is high at about 540V, much lower in the YBA-1 at 400 and change. Measure the plate voltage WITHOUT TUBES.

The idle current that is safe for the tube is based on the tube plate power rating and the plate voltage present in the amp. An EL-34 is rated at 25W. At 400V the current that causes 25W of dissipation is 62.5mA, where at 500V it is 50mA. This shows right away that there is no "ideal current" for a given tube that can be used universally.

Generally, you can operate the tube at its rating forever provided you get the heat away. Most amps idle the tubes at a lower level, typically 50-70% of maximum. This has no bearing on the maximum output power you will get, just on the tone. If the tube red-plates it is dissipating far more than its rated power and will damage itself and the OT in short order.

Ideally, you would modify the raw bias supply to have two pots, one for each tube. This allows you to set the first tube by meter so you know it is safe, then set the second tube by ear for minimum hum which is also the point where the OT has its best bass response.

Our BMK Bias Mod Kits include the current-sense resistors, meter jacks, bias pots and wiring for safe and externally-accessible bias monitoring and adjustment.

Recapping the amp would not be a bad idea, but DO NOT use can-type caps or axials. use modern radial-lead caps mounted inside the chassis. Disconnect the old cans but leave them in place to retan aesthetics and throw away those axial pieces of junk. Those were the way of the day but that day has long passed.

Have fun
 
Because the Standby switch is between the HV center tap and ground, the bias voltage will be quite high when the Standby switch is open. Not even close to what the bias will be in Play mode. That 150K 2W resistor will be quite hot in standby mode, it has about 1000V peak across it. Those old carbon comps almost always turn brown and go high in resistance.

Don't operate the amp without preamp tubes installed. The B+ in the preamp will go to 540V and kill the 450V preamp filter cap. Later versions of the amps had a bleeder resistor to limit preamp B+ when preamp tubes are removed or before they warmed up if the amp was turned on without using the Standby switch. It's normal to see these amps red plate slightly or if you look close, you can see the screen grids glowing orange. They need the fan. Replace it when it goes bad.