Wacky bias readings

Hello All,

My VTA ST 70 is my back up amp, and I have been using it more lately. The issue is that I'm getting strange bias readings, i.e. (new) tubes will bias up to 400, then later drop to around 350 (or less) and refuse to increase, even at the top of the bias pot. Sometimes (new) tubes won't bias up beyond 250, or even less despite the bias pot setting.

There seems to be a 'central' issue. I've been getting more mechanical noise from the (original) power tranny. Could this be a PT failure, or perhaps rectifier? Any help appreciated.

Thanks,

K.

you don't say what kind of tubes. If you're getting a mechanical hum from the transformer, likely these tubes are pulling more current (are you using KT88 instead of EL34?)

To be clearer.. this is a vintage amp with the VTA updates. Basically everything is recent except the iron and tube sockets; quad cap, boards, caps, etc. The drivers are 12BH7/12AU7, and the power tubes are either (new) 6L6 or (new)EL34, not KT88.
Up to now, the amp has worked fine since I did the VTA updates about 5 years ago. The tranny doesn't run hot, but the mechanical noise has been a feature for quite a while, and I am told this is often the case with older amps. The inability to bring the bias level up above 300 - 350 is new. I haven't had a chance to do voltage checks yet, but likely will this w/e.
Just wondered if there might be an obvious culprit to direct my troubleshooting.
If not, I'll check the voltages and report back.

Thanks in advance,

K.

PS: Rectifier swap results in no change to bias readings.

check the actual bias voltage at pin 6 of an output tube.
You should be able to vary it from around -30vdc to -45vdc.
If you can't get the bias reading (which is across the 10 ohm cathode resistor, pins 1 & above 0.30vdc
then the actual bias is too negative, which means R39 is too small. Usually 6.8K on a VTA70 (7.5K on some older ones) and usually 1K on a VTA120 amp.
Or maybe your tubes are just worn out. Never hurts to have a couple of new spares on hand.

http://www.leedsradio.com/images-sockets/Testsocket-octal.JPG

Guys and gals - I have a set of these in various sizes.

They allow me to test voltages in-situ, without opening the chassis or probing amongst all sorts of other live wires.

And are perfect for the sorts of tests described above.

What rectifier are you using SS or 5AR4? If tube then a low HT will cause lower than normal bias voltages.

If you pull one set of output tubes (or wind the bias pot full -ve so they conduct less) does the bias on the other pair alter significantly?

Hello,

Voltage check reveals low voltages across the board; average 100v. low at the tube pins, rectifier and quad cap.
I was able insert power tubes on one side (plus rectifier) and bias, but when adding the other power tubes, they wouldn't go above .30v.
The bias circuit appears ok, as it will swing bias from -20 to -40 v.

Thanks K.

PS: Tubes are new, and switching rectifiers doesn't appear to make a difference.

Hi Bob,

Thanks for the reply.

Hard to be clear on every detail here, but yes the quad is new, along with the addition of the full VTA upgrade 5 yrs ago.

--The transformers, choke and tube sockets are about the only original parts..
As noted, everything's been working ok up 'til recently.

Are there specific diagnostics that will help pin down the cause of the low voltage readings?

Thanks again for responses.

K

I have a VOM that will check the quad cap.
I'll also check the choke!

Thanks,

Kevin

Dingojazz wrote:I have a VOM that will check the quad cap.
I'll also check the choke!

Thanks,

Kevin

I have a Fluke VOM that has a capacitance setting. And is generally useful for simple checks. Some things to know:

The typical standard-tolerance electrolytic cap may be -20%/+100% and still meet "spec.".
Electrolytic caps in the earliest stages of failure may read high - giving a false sense of 'goodness'.
The higher the operating voltage, the more prone such caps are to false readings.

Only an ESR meter, and/or a cap checker that tests at operating voltage is truly effective. BUT! If you have a known-good cap to test against, you can note any differences in behavior.

As always, make sure any cap is fully discharged before testing.

Good luck with it!

I kit-built a Hafler DH-200 kit right after they came out a century or two ago. It was noisy as hell, both mechanically and through the speaks. Somebody suggested I might put a brick or concrete block on the cabinet, and QED. Another example of KISS.

deepee99 wrote:I kit-built a Hafler DH-200 kit right after they came out a century or two ago. It was noisy as hell, both mechanically and through the speaks. Somebody suggested I might put a brick or concrete block on the cabinet, and QED. Another example of KISS.

Not to change direction of the thread, but I had the same problem with an early Hafler DH-200 (one of the first kits to arrive in Southern California).  Wrote Hafler about it and the sent me a copper strap Faraday Shield with instructions for installation.  Mechanical buzz/hum fixed!  

-DB