Tube Amp Archive

With 2 EL34's you should change the output impedance. So if you have a 16ohm cab, set the amp at 8ohms (I hope I've got that the right way round!)

And here is the Mullard, Blackburn site .... no rejects here

Carbon comp resistors=noise. Changes in tone after replacing them = cold solder joints, dirty 50 yr old component leads, your AC kicked on and dropped your house AC by 5 volts. If you haven't tested it, 5 volts difference in ac supply change makes a noticeable difference in tone. I defy anyone to b...

Thanks Phil_S that's what I intend doing.

A lot of the original carbon comps in the amp are over 120% of original value. I'd normally change those as a matter of course. There's also an original big 0.01uF Ceramic disk which is completely microphonic.

Last update. Soldered it all back together. B+ at 450V, output tubes bias range OK, biased cool at 35mA. Noticeably less hiss and electrical noise, though there is still too much hiss. Can't crank it up this late but guitar sounds pretty sweet and full at low volume, bottom end has returned, not the...

Great news! Looks like you'll have a spare PT! Regarding volume pots, the original Fender audio taper for these was ~30%, so linear (i.e. 50%) is as close to that as most log types (10%). Perhaps the pot is knackered, it really is on or off, full blast at 2 and no more volume after. I've got some n...

Hooray for us! Your B+ should now be much closer to the values on the schematic. Congratulations and we should give a prize to pdf64 for his determination of the real issue. This isn't the 1st time we've seen something "professionally repaired." Indeed, many thanks to you all but a special thanks t...

Bingo! I just lifted the yellow wire feeding the bias form the board, used it as centre tap and yes, getting 333V each side of the HT secondary. I'll just check the bias voltage now, but it looks like it's been a PT wiring issue. Hasten to add it came to me like this ... after being "fixed" (it was ...

Ok so taken the PT HT leads from the diode string and measured direct to the HT section centre tap. Lead 1 390V Lead 2 280V So is there a bias tap on the HT winding, as well as the centre tap? Might they've been connected the wrong way around? The bias voltage is reading 53V AC before the 470 ohm r...

Besides the NF mod these Amps have a bad sounding PI section in and of itself so that would be the next area to attack after you resolve this possible B+ issue! Someone has changed some of the PI values already, but to be honest the amp just sounds nasty, most likely through the B+ problem. In this...

Ok so taken the PT HT leads from the diode string and measured direct to the HT section centre tap. Lead 1 390V Lead 2 280V Consistent at least! Looks like it is the HT section of the PT at fault? The PT does vibrate horribly when the amp is fully on, tbh I'd expect smoke at any time! I've got a pro...

Many thanks for all the advice, much appreciated. So I've re-measured using croc clips and the standby switch closed (operating) and allowing a few seconds for readings to stabilise. Across both HT wires before diodes 670V HT supply wire 1 388V HT supply wire 2 282V Bias tap 53V Heaters 6.3V B+ afte...

Thanks for your replies guys. Ok, so I've put a new battery in my meter and re-taken voltages with standby off as suggested (I had measured with standby on before) The first thing I noticed is that if I measure the AC from the PT at the start of each diode string I get 276V at the lead for the first...

Hi recently got an early 1968 Bassman as a fixer upper, though it does work. Circuit has been AB165 but it's had the usual mods to NFB and has proper bias adjustment. I'm in the UK. The amp has originally been a USA non-export model and the transformer is not original ... but I don't know if it was ...

V1 is a challenge. Pretty certainly a Philips made in Eindhoven (the 8 part of the code) beyond that can't find anything that works with those numbers that looks similar. Here's a link to a great resource for Philips codes. http://frank.pocnet.net/other/Philips/PhilipsCodeListAB.pdf Good luck! V2 is...