Help in removing master volume from silverface Twin

[ctcpete]

Hello, I'm in the final stage of refurbing and blackfacing my Twin (aa270, 100W) I am stuck on the last step, removing the Master Volume feature. Has anyone removed the master volume feature from one of these circuits? I have removed all the wires coming off the Master Vol pot from the circuit and have been comparing the aa270 silverface and 763 blackface circuit which are both attached to this thread. I'm having trouble figuring out how to proceed from here.

Any hints would be greatly appreciated.

[ctcpete]

link for the master volumd circuit

Here is a link for the master volume circuit, it was too large to attach the pdf.


http://http://www.ampwares.com/amp.asp?id=114

[52 Bill]

The master volume control is set up just like the channel volumes are. The signal coming in attaches to the top of the pot, the input to the next stage attaches to the wiper, and the other end of the pot attaches to ground.

Find the two 220K mixing resistors that sum the output of the two channels (wired to the top of the old pot) and wire them to the input capacitor of the phase inverter (wired to the wiper of the old pot).

[ctcpete]

Thanks Bill,

I'll try that next time I get access to my "workshop" ie: the kitchen table when the wife isn't around.

Thanks again for your help. You've given me good advise in the past as well.

Regards, Pete

[ctcpete]

Just to clarify..

So I only need to run a wire from the junction of the two 220 k resistors to the input cap of the phase invertor? All connections coming off the master volume can be eliminated? I'll leave the physical pot and knob in place with no connections to it.

Thanks again. I'll report back some time next week with my results.

[52 Bill]

So I only need to run a wire from the junction of the two 220 k resistors to the input cap of the phase invertor? All connections coming off the master volume can be eliminated?

Yes, but if your amp has a push pull switch, those wires will have to be dealt with by removing the associated circuitry.

[ctcpete]

Yes, it does have the push pull switch...

So it sounds like I won't get away with simply removing the wires from the pot and connecting the two points that we discussed earlier. I did label each wire from the pot and noted where they originated before removing them.

I was planning on leaving the Master Volume feature as is but yesterday I replaced the two 330 k resistors right after the phase invertor cap and master volume pot with two 1 meg resistors (blackface values) I also replaced the existing .01u cap at the input of the phase invertor with a blackface value .001u. immediately afterwards when I tested the amp I noticed the master volume pot seemed to be not working right (dead spots and noisy) which I never noticed before. I tried spraying some De OxitD5 into it which didn't help. I then assumed that the pot was NG and decided to just remove the thing. Now that I look at the Master Volume schematic, I wonder if the values of the Blackface components (the .001 cap and 1 meg resistors)where not compatible with the Master Volume feature causing the Master Vol pot to appear to be defective. I'll try to attach the schematic, it may be too large.

[ctcpete]

attachement for Master Vol circuit

Here is the schematic for the master volume circuit, it show the push pull switch in the "pulled out" position

[ctcpete]

oops! I found a mistake...

After looking at schematics and then my chassis, I discovered a major brain fart on my part. The common connection where the 3 resistors (two 330 ks and a 470 on the silverface version)just before the phase invertor meet , they should be connected to a 22k or on some schematics a 33 k resistor. I used the wrong connection point (an unknown solder point to the right). I've been doing the work while watching my 5 year old twins so I'll blame the distraction factor. LOL.

After I connect to the correct point I am sure my results will be better.

Any thoughts on if I should leave the master volume connected as it was, or remove the entire function or just the push pull feature. Any pros or cons?
Thanks in advance.

[ctcpete]

Update, back in business..one more question...

I figured out what I had done wrong during my blackfacing work. After correcting my work, I restored only the Master Volume connections to the circuit. I removed all connections (at the pot and at the circuit connections) coming from the push pull section of the Master pot. Is this an acceptable way to remove the push pull feature?

The amp now sounds great, I've biased the new power tubes and left the original RCA pre amp tubes installed
for now.

Thanks for everyone's assistance.

[bnwitt]

What year is you amp? Was you MV control a push pull or just a standard pot?

[ctcpete]

The amp is a '74...

It had the Master Vol pot with the push puill feature. I want to leave the master volume aspect as is and remove the push pull feature. I removed all the cables (at the pot and the circuit) that control the push pull (the connections that come from the green section of the pot) the Master Vol works fine this way.
My question is if just removing the push pull cables the correct way to go or is there additional circutry that should be removed (Bill52 had mentioned this earlier) to correctly disable the push pull feature?
Thanks.

[52 Bill]

My question is if just removing the push pull cables the correct way to go or is there additional circutry that should be removed (Bill52 had mentioned this earlier) to correctly disable the push pull feature?
Thanks.

Removing all of the cables is be the same thing as having the push/pull switch turned off.

I mentioned the push/pull circuitry earlier because you said that you were "Blackfacing" this amp. To me, among other changes, that would entail removing all of the push/pull circuitry as well as the master volume control.

If you're happy with the sound then you're good to go.

[ctcpete]

Thanks Bill. Is there any benefit in removing the push pull circuitry tone wise? If there is I could try and compare the two schematics and try to determine what would have to be pulled.

[52 Bill]

Thanks Bill. Is there any benefit in removing the push pull circuitry tone wise? If there is I could try and compare the two schematics and try to determine what would have to be pulled.

There are two areas that contain the push/pull circuitry. One is at the reverb driver transformer output, where there is a 1K and 470K resistor added. Removing these parts will only effect the signal going into the reverb tank.

The other area is at the input to the phase inverter. There is a 12K resistor in series with an 0.01 cap, in series with a 1M resistor. These may load the input to the phase inverter a little, so removing these parts may make a little difference.