volume problems with a first amp build

[deaththreat]

i have built my own 100w tube amp and have bean doing a lot of modifing on the design and have fixed other issiues that have risen but for some reason i am now stumped because it never has done this before wich is at lower volume the amp sounds great except a further increase in the master volume the sounds loudness fluctuates up and down in volume which before it had cranked without these effects can anyone please help thank you , chuck

[loudthud]

Describe the amp. Is it like any production amps? What tubes, how many preamp stages, control configuration, etc... Also, what is your general experience level and what do you have in terms of test equipment?

Got a DVM? Connect it across the speaker terminals, set it to AC volts, turn the master vol to where it makes the strange sound. Anything there when no guitar is plugged in?

[melvin]

It wasn't doing this before? Did you have it working well and then made some changes? A schematic and maybe a picture of the layout would help. Bad layout can cause this problem. Other things as well....

[deaththreat]

it is an amp based off a boogie

this is the layout of the amp . i have not had the problem until now. i would say intermediate level in electronics. i have a dmm,wave generator,variable power supply,and an oscilloscope . i tried going in with audio gen from computer and the problem does not happen. i was thinking it could be the output transformer going bad which i had salvaged from a carvin 100w head that went out on me that led me to build this amp but i do not know how to go about testing the output transformer any sugjestions? chuck

[loktar]

First of all, you should locate where the problem is.
1. Try to plug bypassing the preamp.
2. Measure bias - sometimes such an effect comes from floating bias.

[melvin]

what kind of master volume do you have? can you bypass it? Are your output tubes getting all their voltages and have you looked at the current draw from each side? I wonder if these things might give a clue...

[guitician]

How clean is that PC board? Did you coat it right after etching it? High voltage just loves to run along a surface, if given a path. You may have sub-sonic oscillations going on where current starts to flow in a path across traces. As the voltage drops it stops, but then, it rises back up and restarts. But, then you could have a partially fried OT.

[deaththreat]

thinking same thing

thinking same thing about the ot because it did start smoking from heating o.t. once but i still have output and has not smoked since but does this mean it could still could be bad and if so how do i test it ?
chuck

[guitician]

As the primary's insulation breaks down it starts to loose it's inductance and the carbonized insulation may start to conduct and arching between windings will cause more damage. This usually happens when it is been heavily loaded playing at full output. If you have all the technical specifications, and can measure yours, and you may find it has lost impedance. Play an audio sweep sine wave under full power resistor(200 watt) load before clipping, is it still a clean sine wave? What does it look like when the volume decreases?

[deaththreat]

i am going to check

i think i might have found the problem i am goikg too look at later plese tell me if this would cause problems . i have the A and B +V's from the power supply to the main board wires wraped in (shielding)that is then connected for the supply and main amp boards ground connection? chuck

[guitician]

If you see and high frequency oscillations on the scope, then that may be what is causing it.

[melvin]

I am not sure that it's going to make a difference, but it's good practice to only ground one end of the shielding, and high voltage or current wires don't need it. Furthermore, if you are using the shielding as your ground connection between boards, the capacitance of that arrangement could indeed be your problem...

[deaththreat]

well i just finished 1 idea

it looks as though it did not take care of the problem but it did help clean it up better so all was not lost . i believe it probably is the ot going out and as soon as my next paycheck i guess i will buy a new one as this would not hurt anyways it is i would say its mostlikly worn out as nothing last for ever and when i do i will let you know how it turns out regardless of out come and i appreciate your help ty, chuck