zulu said:
Output transformers, inter stage coupling caps ... I've heard of people using/replacing tone caps in guitar wiring with those PIO caps.
Hi Zac: the OTs are the two, smaller rectangular chunks on the tube deck .. more on them in a minute. Below is a rough match between the schematic and your amp ... not guaranteeing that I've ID'd all of the coupling caps in the photo:
Brief lecture: downstream of the rectifier, all the power in the amp is DC ... the musical signal is AC like the typical sinusoidal waveform you see on an oscilloscope. Except for the trem section, each successive stage that the signal passes through amplifies the signal using DC power. Having been amplified, the signal is passed through a coupling cap to the next stage.
The reason a straight wire isn't used is the presence of DC. The AC signal passes through the coupling cap but the cap blocks the DC ... otherwise the following stage ... and all the stages thereafter, would receive and amplify the DC and that produces a highly undesirable screech. So ... what's needed is a way to connect stages but block DC ... interstage coupling caps. The term 'coupling' refers not to type but to application.
Yes; PIO caps are popular in guitars as tone caps. One of many properties of caps is that, depending on size/uf value, they are frequency selective; small uf caps pass high frequencies but not lows and large uf caps pass low frequencies but not highs. In a guitar, as the tone cap is rotated from treble to bass, more and more high frequency is shunted to ground because it - and not the bass - pass through the cap because the cap is a smallish value; that is to say you aren't adding bass, you are bleeding off treble.
Since you only need 2 PIO caps for a two-pickup guitar (don't have to spend alot), lots of people put them in for the same reason they put them in amps ... the belief that they'll max out the tone ... as do the boutique amp makers like Clark and the others who use Sozos, Jupiters, and ????????? ?????????? ????? (various Russian brands). To be argumentative for moment, if those polycarbonates(?) in your amp and the Mallorys I use were so well thought of by the high-end crowd, they'd be using them and not PIOs, no?
The biggest risks (apart from burning your finger or nicking nearby insulation with a soldering iron) are that after so much work, your expectations aren't met and it's arguably 'violative'; you can justify all your moves to date as heavy maintenance, go further and anyone looking for a stock T1 RVT won't consider your amp or, if they do, discount it no matter how good it sounds.
The OTs: the reverb transformer is easier to find that the primary ... owing to those 6GW8s and their oddball internal resistance. The primary OT would probably have more affect on perceived volume and clarity although if you're going to do one, you might as well do both ... I mean if you don't but decide later you want to, it means dragging the chassis out one more time.
zulu said:
I am itching a bit to start the Ampeg. Goal would be to use as a guitar amp (6L6 mod?), paired with a 2x12 cab (output step 16 to 8 ohms?), for stage use with effects pedals. I'll start a new thread soon with a rundown on the problems with the amp (ie, always ran it with 8 ohms or less load, oops), and I really appreciate your offer to help with it.
Ok; sorry ... it's been so long ago ... B25 / 7027s? Whenever you wish.
Stoly .... although Absolut is ok too (burp :wink: ) J