Another Asteroid/K60 Build

[martin manning]

You may have a leaky coupling cap. If you disconnect the bias supply from the grid resistors is the bias voltage steady?

[deuce42]

Ok now this amp is really driving me nuts!!!

I've changed caps in the bias supply and still I cannot seem to lower the amount less than 42ma. Plate minus cathode voltage is 484v. If I measure pin 5 without the tubes, at the lowest point I can get it is a stable -55v. Cant seem to reduce it to -40v. It also seems to be that however I move the trimpot, the voltage is only a few mv different from one extreme to the other.

I really want to get to about 32ma bias. I changed the 47k resistor from bias pot to ground to an 82k. Tried 100k as well. No change.

I just don't get it....

[M Fowler]

Something is not right, normally on the Asteriod with EL34's with plate voltage 484 it will bias around -45v.

Bad PI tube 12AX7 or bad 5881's plus check for leaky coupling caps like Martin said.

around 27k to 33k to ground from bias pot is normal.

I use the stock 50kL multi-turn bias pot for Asteroid amps.

Mark

[deuce42]

Thanks Mark

I've changed every tube in the amp and replaced all the electrolytics for good measure. I've still got the same problem no matter what I do. My only other suspicion is the transformers themselves?

There seems to be a strange squealing noise coming from the xformer itself (although I cannot hear whether it's the power transformer or the output transformer making this noise). I also note that as I turn up the volume control the bias milliamp reading goes crazy - up to 80ma.

As for the noise on the xformer, it sounds like a buzzing sound and then turns into a high pitch squeal raising in pitch. I was reluctant to keep turning it on to test the amp, lest I cause (or have caused) permanent damage to one of the xformers. I can tell its the amp making this noise because I've connected a reactive load to the amp rather than a speaker and can hear the squeaking sound.

I'm just thrown by this circuit. I just cannot ascertain what I'm doing wrong. Any help would be so appreciated...

Thanks

[martin manning]

Sounds like you have an oscillation going on there. PPIMV lead dress can cause that. Do you have them shielded and routed carefully? Can you scope it? Have you tried disconnecting the GNFB?

[deuce42]

Thanks Martin

Would the PPIMV lead dress cause the actual amp itself (ie not the sound from the speaker) to make this squealing/squeeking noise?

[martin manning]

OT's are often heard to make sound when the amp is connected to a dummy load. It's odd that you hear sound from the OT but not from the speaker... presumably this oscillation is ultrasonic. It could be that the OT is producing sound from a harmonic.

[deuce42]

Ok so I will try to redo the leads on the PPIMV to see if there is a way to ensure there is no interference. As for the still crazy biasing, I just cannot ascertain what else to do. The biasing just will not get to a stable 30-35ma no matter what I seem to do. Also the fact that the bias milliamps go crazy with turning up the volume seems strange to me unless I'm doing something seriously wrong.

[martin manning]

Ok so I will try to redo the leads on the PPIMV to see if there is a way to ensure there is no interference. As for the still crazy biasing, I just cannot ascertain what else to do. The biasing just will not get to a stable 30-35ma no matter what I seem to do. Also the fact that the bias milliamps go crazy with turning up the volume seems strange to me unless I'm doing something seriously wrong.

This behavior suggests an oscillation, which is ultrasonic since you aren't hearing it. If you turn the PPIMV all the way down (basically grounding the power tube grids) does the bias current remain stable? PPIMV leads need to be shielded or twisted.

[deuce42]

Thanks Martin, have tried so many different combinations and still stuck. Have twisted the wires of the PPIMV very tightly and tried to ensure they are nowhere near anything else until the hit the 820ohm resistors going to the power tubes.

I get the following measurements:

With EL34's and a 250ka PPIMV with 2m2 resistors :

Volume at 0 and PPIMV at 0 reads 6.3ma to 15.5ma as I sweep bias pot from one end to another. It is stable.
Volume at 0 and PPIMV at 7 reads 15ma stable. Push past 7 on the PPIMV and it instantly doubles to 30ma. Once PPIMV is on 10 its about 41ma. A bit unstable, ie jumping around between 1-2ma either side.
With the PPIMV on 10, as I increase the volume the current increases dramatically and crazy. With the volume control on 10 its about 110ma.


With 5881WGC's and a 250ka PPIMV with 2m2 resistors:

Volume at 0 and PPIMV at 0 reads 23.2ma to 35.8ma as I sweep bias pot from one end to another. It is stable.
Volume at 0 and PPIMV at 7 reads 23ma stable. Push past 7 on the PPIMV and it instantly doubles to 33ma. Once PPIMV is on 10 its about 43ma. Reasonably stable, however the bias pot barely does anything. Ie turning the bias pot one side to another barely changes the current be 1 or 2ma.
With the PPIMV on 10, as I increase the volume the current increases dramatically and crazy. With the volume control on 10 its about 110ma.

I wanted to swap the PPIMV for a 100ka with 2m2 resistors but since I'm having such a difficult time with the existing PPIMV pot I thought I'd leave that for now.

I cant workout what else to do. Could the output transformer be having problems instead?

Im sorry to be such a drag and waste everyones time. I swapped out different tubes as well. I ultimately just want to use a 100ka PPIMV and 5881 tubes. I'm not sure why this is going so badly.

Thanks again:)

[martin manning]

What about the negative feedback? Have you tried disconnecting that, or swapping the OT primary leads?

[deuce42]

Tried removing the negative feedback but no change there.

Tried swapping the primary leads on the output transformer - still the same problem, with EL34's I cant get more than 15ma with tubes maximum biased and with 5881's I can get in the 30's but as soon as I get the volume past 7 it jacks up almost instantly to between 75-100ma.

When I use a 250ka dual pot for the PPIMV things don't squeal with the dummy load, but when I use a 100ka dual pot the amp makes a bizarre oscillating noise - I cannot tell whether its emanating from the output transformer, the power transformer or the power tubes. The PPIMV leads are twisted extremely tightly.

I'm truly just out of any more things to try. Swapping tubes does nothing either. Could the output transformer be busted?

Is it time to accept defeat and just buy an amp.......?

[wpaulvogel]

Have you put a bottom cover on the chassis during the test. These amps are so sensitive that sometimes a complete faraday box is needed.