Tube Amp Archive

Anyone can help?
In particular I'm in difficulty to confirm if the output signal of the 12AX7 (or the 12AT7) Phase Inverter, considered in the Marshall Silver Jubilee implementation, can feed correctly the pair of 807 (in the Geloso implementation)?
Or what kind of modification are needed.
Thanks.

Racing on vintageamps.com/plexiboard/ did a lot of Geloso transformations - maybe the wayback machine is a way to catch that info. How technical are you - there is a lot that could be done with that type of restoration, especially related to the powering and biasing, and working through all the des...

Hello everybody. For your information and to close this thread: all my issues were due to a conductive board. In the last times I worked a lot on the issue of the coupling cap that had a strange 700mV on the side were near 0V is expected. Replaced it many times, during operation I try to measure the...

I spent few our in checking the connection to ground, including desoldering and resolder the Presence part, changing the 100n cap. The situation doesn't change. On the PI I've 217Vdc on the pin1 and 240Vdc on pin 6. One thing I noted are the Vdc voltage on the NFL line: -2Vdc before the 100k resisto...

Sorry for the wrong information but the voltage on pin 1&6 of the PI are dropped also without any input signal. Now they are 200Vdc and 215Vdc.
And when I touch the pin 6 of the PI with the scope probe the volume in output (I have a speaket connected) raise up.

At this point only confusion

In addition I tried to apply a sine wave (1kHz) to the Low Input and look what happens to the PI plate pins:
- the VDC voltages drop around 30vdc on both pins (pin 1 goes to 200Vdc and pin 6 goes to 230VDC)
- on the scope the signal wave is around 20% higher on pin 6 than pin 1

Hope this helps.

The amp sounds good as tone, but seems not reach the full power, the sound seems compressed after the volume raise after 12. Looking at the phase inverter issue I replaced the 100n cap on pin 7, connected to the NFB. And to be sure I replaced also the 2 plate resistor 82k and 100k and try to unsolde...

Checked also both grid and cathode resistors: they are ok.
I tried without the power tubes. Here what’s happen:
- Pin1 = 230Vdc – Pin6 = 255Vdc (a little better)
- Pin 2 & 7 are not more exactly equal  Pin2= 32Vdc – Pin 7 = 33.5Vdc
Suggest something?

I worked a couple of hour of issue 2), but with no success. I changed the tube with others, but the different voltage remain between pin 1&6 --> pin 1 vary from 210 to 220VDC and pin 6 vary from 250 to 260VDC with different tubes. With different tubes when pin 1 decrease it seems that pin 6 increase...

Hello guys. I recently found this old (around 1960-65) Geloso 100w push-pull amplifier and had the insane idea to transform it to a guitar amplifier. Basically I'd like to mantain the power part and rebuild the pre using a Marshall schematic (I'm a Marshall fan and love the sound of the Silver Jubil...

Successfully substituted the TL072 and the amp works quite good. There's a residual hum using the high channel, but playable in any case.
I wondering if there is a simple way to protect the op from overvoltage/spikes that may be generated by the insertion of the jack on the amp input.
Any suggestion?

Hello everybody. I the last couple of week I worked hard to renovate a JCM800 2204 50W clone I bought some years ago. It was modificated a lot by the previous owner and I'd like to restore the original Marshall components (my bible was the schematic of Mark Huss JCM800 Lead Series 2204). I substitut...

Thanks for suggestions, Stevem.
What do you think about cutting the pins on the component side and solder the new OP to them
Any other suggestion about what can generate this strange Vdc on pin 1/2/3?

Hello everybody. I'm a newbie in amplificators repair, but hungry to learn. My (little) experience is more on tube amps, but now I'm working on a Marshall valvestate 8040 that is an hybrid machine (attached the schematic). If I attach any cable to the input, also without signal, the amp become very ...