The Amp Garage

Hi, Try lifting the newly fitted PI caps from the bias side up in the air. Run wires directly over to the O/P tube grid resistors. Not pretty but it will ensure that there is no longer any contact with the PC board on the bias side. I found that there mustn't be any contact with the PCB on the bias/...

Hi, I have some notes re the BTR23 amp I had in for repair many years ago. Single cap boost across input cathode resistor and a low value 100K MV after the tone stack. The low MV value, in my opinion, gave it a unique sound and seemed to choke the TS function. (In a good way) This plus a heap of oth...

Hi, I did a PM to Alexander with a hand drawn schemo of the Pro 35 (2011) that I did several years ago. It was very rough so I thought I would alter his schemo to reflect the differences that I found when I copied mine. I'm not saying mine is without errors, but present it as a possible alternative ...

Hi,
Have you swapped out the the tone stack 250pF treble cap ?
I have found that if it is a brownish colored Silver Mica cap, then they can become leaky.....I change these mica"s out on spec now days especially if it is used in a high voltage situation.(plate connection)
My further .02 cents
John

Hi, Two things, with the amp chassis exposed i.e out on the bench, just your body and other devices in close proximity can induce hum. Re noise I find most times in MB amps that after all the tubes swap outs and anode resistors changed, there is often still varying amounts of residual hiss, and usua...

Phil,
I hope Mr Frey won't mind me posting his schemo.
For comparison purposes only.
Yours is missing a source resistor in the send FET.
All the best with solving this problem
John

Phil, Have you made sure that there are pull down resistors fitted at the input and output of the Friedman loop. A couple of un terminated jack plugs inserted into the respective sockets then measure resistance across each jack just to make sure they are fitted, If they are (220k send and 2M2 return...

Gillies,
Tried to PM you.......didn't work ??
John

Ask Mark if he remembers this..................
John

https://www.youtube.com/watch?v=QeIxJzdPD0A

Hi,
Just measured a VOX AC30CC amps HT winding resistance and got 52 ohm and 50 ohm with respect to the centre point.
Looks like I will be installing 47 ohm resistors or there abouts in each leg to the GZ34 Anodes from now on.(maybe the diodes as well)
John

This time.............

Hi,
Aside from manufactures exceeding the MAX filter capacitance, some ignore the minimum HT secondary resistance. ( see Mullard spec) I must measure the VOX AC30 CC secondary resistance.
John

Martin, Thanks for correcting me. I install these diodes in just about every modern amp that uses a GZ34 rectifier. The most common being the Vox AC30 CC where both the Sovtek and JJ tubes blow often. Since installing the additional SS diodes the GZ34s longevity has been greatly extended. Maybe we a...

Hi,
The series diodes before the tube rectifier should help limit inrush current.
John