even in an era when people had hi-Z mics on unbalanced quarter inches and no stage monitoring for the signer this amp was an AWFUL design for a PA. Though its a loud and stout 50 watts it is not a very clean 50 watts. Mine is the 1st gen circuit from 68 or 69. It utilizes the not-so-hifi concertina phase inverter and has not-so-hifi grid leak bias V1 and V2, the 4 triode stages that serve as the 4 individual mic/line inputs. It has a hi Z effects loop after the 'master volume control'. Pretty much goes 4 channels with 1 stage each, individual volumes, master volume, send and return, then a driver stage for the 2 band EQ followed by a driver stage for the pahse inverter and then the pahse inverter driving the EL34 woer section. Solid state diode bridge rectification and a wide ranging presence control that's very british ind esign round out this amp. Awful PA design but if you made it a 1 channel it would look an awful lot like an Orange/Matamp. Scarily like it actually, just without the FAC control. For guitar this amp sounds like an early Orange ehad. Drive it into dirt and its just osntrous. The clean sound is complex and punchy. Hifi yet smokey. British but unmarhsally. Cops some hiwatty tones too due to the fast response of the wonderful old Hammond transformers which are WAY oversized.
I got this and I thought it would eb really hiwatty like the 4 input Pete Hiwatts due to some bad internet info but its more like a 4 input orange and that's okay. More than okay actually. These can be had for peanuts. They only made this circuit for a year before changing the EQ to be more fender than baxandall, switching to a fendery long tailed pair inverter and dropping the psot EQ recovery stage. With these changes it probably made a slightly better PA by the standards of the time and I'll bet it made a MUCH better bass amp (its secondary role) but probably is an anemic and wimpy guitar amp for rock n roll.
The original baxandall/james type tonestack is awesome by the way. there's like 1 perfect setting for each guitar. it took me awhile to get used to since its not quite an ampeg bax EQ and tis not precisely like a 2 band vox EQ either, total mutt that's highly interactive but not in a way that's immediately intuitive.... lots of control of the mid content from those 2 knobs though! the presence sounds the most rocking all the way up but good tones can be had throughout its range if you dial in enough trble.
If you want something like this look for a '69 or earlier. Talk to the guy about the transformer date codes and ask for gut shots and analyze them with the help of a tech if needed. Mine is a rock monster that conjures up powerful 70s muscle car tones, but YMMV. if you get the right one it will SLAY! I have JJ 34s in mine right now but at one point I had it biased up for JJ's very robust 6V6 and the sucker gave up the pwoeramp distortion like immediately and put out maybe 25 watts RMS .... great for recording but it didn't have the stout clean tone I like from this amp so I switched back to 34s. Came stock with either 6CA7s or those 7027A mil spec 6L6es that ampeg liked to sue. As those dried up in Canada where it was made traynor switched to the CA7es which are just a beam tretrode, USA made EL34 more or les.... basically the American KT77. With those tubes or a 6L6 the amp will have less midrange and more thumpy lows/glassy top. I think tis best with EL34s, though I might try 77s too sicne they're kinless and will deliver the mids. I generally find the 77s to be too clean though.
Interestingly there is an unused gainstage on V3... to use it without introducing nosie you would have to heavily modify the power supply making a separate line for that stage, but you couldalso run it off the same line as V3a with minimal supply noise as long as you weren't trying to make an over-the-top gain banger outta her, justa dd little more gain. All I did to mine was to disconnect the old amster volume and set it aside and then wire an Orange Overdrive model post phase ivnerter master in tis place with a dual ganged 1m pot hooked up between the 2 outputs from the splitter and the 2 output tubes. This is an effectice little control that's been utilized on long tailed inverter amps by matchless and more recently vox and its nice here though I typically leave the control wide open and dial in a loud, edge of breakup tone and then use a dirt box out front if iw ant more hair.
anyway, love this amp... hard rock beast and secret weapon