champ style build problem

[wsaraceni]

i built a champ style preamp into an ax84 power amp/ power supply. it seems like the gain stages are putting out too much gain. how do i back off the gain on each stage to tune it to where i want it.

[PRNDL]

The ax84 power amp is a LTP PI and push-pull 6V6's, which is pretty standard.
They also have several standard preamp sections.

What do you mean by Champ preamp?
Is there one or two stages of gain?
Is there a tone stack or volume in between the stages?

Also, can you describe the problem?

The power amp can be driven by as few as one gain stage (1/2 a 12AX7), which would be equivalent to a Matchless Spitfire or Vox AC15. Breakup usually begins at about 4 or 5.

A second gain stage gives more gain and early preamp distortion.Breakup and distortion occurs very early, sometimes at 1 or 2, which is why most amps have a preamp volume pot.

[wsaraceni]

the preamp is pretty close to a 5f1 champ. gain stage, 1m vol pot, gain stage power amp from the p1. the problem is that im getting like full gain at like 1 on the dial.

if you look at this schematic here.

http://annex.ax84.com/media/ax84_m311.pdf

its pretty similar to what they have here but without the tone stack and C12/C16. also the values for the rest of the preamp is closer to the champ values. im currently using a el34 but plan to put in a 6v6 once i get one. almost thinking of ripping everything out and building a straight up champ but i really wanted to get it working with this chassis and transformer set.

[jjasilli]

wsaraceni (http://www.thegearpage.net/board/member.php?u=2293): your question, as stated, is not answerable. You need to post a schematic of your actual amp. A complete set of voltage readings would also be helpful. Otherwise we're shooting in the dark.

If you need a simple program to draw a schematic, and a sample voltage chart, checkout hoffmanamps.com

[wsaraceni]

ill try to post a schematic and voltage chart but if you look at the ax84 p1 schematic, here are teh changes i made.

no tone stack, pin 1 of v4a goes into r9. power amp remains the same

pin 2 of the volume pot goes direct into pin 2 of v4a

pin 8 on v4b to ground is 1.5k resistor and no cap
pin 3 on v4a to ground is 1.5k resistor and no cap

and im currently using an el34

thats all there is to it.

[jjasilli]

Various possibilities:

1. try an input resistor 33K - 68K before R25.

2. I assume you have a coupling cap after the output of V4A

3. R9 is probably superfluous.

4. Without the tonestack and VR1 (MV) you need a grid leak resistor for the power tube: 220K or 470K : The higher the value the more gain.

5. Reduce R25 to as little as 220K

6. Reduce R23 and/or R18 to 820R.

7. If the amp is too dark Install c16 and/or c12, but follow these caps with a series reisistor to gound, as high as 10K. This will restore highs, but limit the gain cause by the by-pass cap. You can make the caps switchable!

8. Replace V4 with a lower power tube like a 12AT7, or 12AU7.

9. Repalce R22 and/or R17 with a split load plate resistor.