Guitar speaker expert from Jensen interview on YouTube.
Have a watch. About 50 minutes

Regards,
Mandu.

https://www.youtube.com/watch?v=ZAUpWQWlTe0

Sorry wrong post

This is not a standard in most guitar amps including Marshall. The low pass filter is set to pass signals below 20 kHz. The drive circuit produces lots of harmonics and may be well beyond audible range. The filter may be part of circuit to produce that Marshall sound for this model or the designer added this to his taste. For guitar applications, Low pass filters are generally provided before and after digital delays or BBD circuits. Regards.

When you replaced Q1 and Q2, probably the transistor pins are not same as the replaced ones, even if the same number. The pin connections some times vary between manufacturers. Regards.

You can get Zener diodes for almost any voltage and wattage. They work as shunt regulators. Don't get easily damaged by over voltages like the 78xx. But voltage will dip on over current by load. Very widely used in many instrument amplifiers to drop from higher voltages above 35V DC. 78xx/79xx are Series regulators limited to selected voltages and prone to damage by higher supply voltages and some tend to oscillate under less load. Application to use 7xxx or Zener depends on the designer. All diode/transistor junctions are noisy when reverse biased. 78xx/ 79xx use zeners internally.

Did you check the 470 mfd emitter bypass capacitor? If low value, then less gain at the very low frequency setting and the oscillator will start and stop at power up. You can temporally place a similar capacitor to test. Regards

The tremolo oscillator is on the right side of your snap photo, called as the phase shift oscillator . You can test if the output voltage on the collector is changing using an analog multi meter. The 3 nos. 1 mfd capacitors, 3 resistors (two are variable potentiometers 3.3K+50K pot) and the transistor form the oscillator. The transistor gain must be greater than 29. The phase shift happens from the collector to the base by 3 x 120 deg. with the 3 capacitors and 3 resistors. Varying one or two resistors changes phase shift and thus the frequency. If the phase shift fail, ex. dried capacitors or high resistance, the oscillation will stop. Regards.

ECCxx and ExxCC may look same, but they have different heater elements.
ExxCC has a 10,000 hour long life filament against 1000 hours for the ECCxx.
ExxCC was considered as industry/military grade tube those days.
Rest of the specs were the same.
regards,
Mandu

If the schematic is similar to what Soundroi quoted, you can need to use the effects out as the preamp out to your other amp or mixer etc. You will also need to plug a dummy phono jack into the effects return to prevent signal going to your randall poweramp section.
Thus you may not need any internal mods on the amp.

regards,
mandu

Shielded mains cables are used in Industrial and hospital environments to prevent interference from mains into the equipment. These equipment usually measure in micro volts of some thing and very suseptable to mains noise. The screen does not help if teh grounding is noisy. They also use clean earth which is separate from electrical earth.
These equipment also have line filters to filter wide band noise.

The skin effect always exists, but very very negligible at mains frequencey. The higher frequency you go, the current at the center compared to the outer conductor goes towards zero. The dead area also increase with frequency. This increases the effective ac resistance (impedance) of the cable. In practice, they use tubular conductors for high frequency or use litz wire for smallar currents- many fine strands of thin wire insulated from each to improve effective current.

There is no need for these cables in guitar amps, unless it is a marketting hype.
Good grounding methods are just what is required.


regards,
mandu

'I ruled out the speaker.  The buzz is present within the note, but isn't there when there is no signal'

Did you try with another speaker? All speakers are quite when there is no drive.

regards,
Mandu

Thank you all for so many guide lines. About 12 years before I have built 20+20 watts class A amp with sucess. For the heat sink I used a VSD chassis. That time I did not have the guitar to test. I gave it as a present to one of my friends. I believe that design was from Elector magazine, with two 2n3055 at the output with upper transistor acting as a current source.
I am looking into a good heatsink now, I may go with a fan cooling if I get a smaller one. This will be all descrete amp. Since mechanical construction is a pain, I believe could be done in a months time. This will be a amp head only, difficult as a combo.

For the preamp, I would like to use all FET preamp.
Thank you all once again.

Thanks for the feedback.
I am not aware of any branded class A amp in the market, but looking into building a 30 - 40 watts amp (pushpull, with output transformer)
I play cleanonly.
regards,
mandu

Could also be
the output stage bias current is a bit low due a component problem.
This buzz usually disappear at higher volumes and returns when the sound is about to die.
OR
the speaker is rubbing on the cone.
You can try with another speaker if possible

regards,
mandu

I never had a chance to use a class A solidstate amp with guitar.
Anybody has experience trying a guitar with such an amp?

Thanks in advance.
Mandu