Saturday, July 16, 2011

ClassicTone transformer on a 1968 Marshall Amp

This is a Marshall Super Tremelo Lead 100 watt head from 1968 

I have repaired, modded or re-stored about 200? or so Marshall amps, starting back around 1971, which means I am older than I'll ever tell, but I will say, I started far enough back to work on, or try on not get killed by all those "nice" 200 Marshalls, SoundCitys, 300 watt Plush, and Orange amps.

I had a 200 watt Marshall on the bench, around 1971, and the customer/friend was standing next to me. 

As the big ones did, this amp shorted out around the 6550s, and both of us freaked, and then somehow, we were both like five feet away, without even noticing it! 

I think the HV fuse blew, and the amp sat there laughing at us

This amp looked okay except for a bad 2nd bias cap. Here I am using a low-voltage power 
supply to charge up the caps and measure leakage current to see if they are good

The one hooked up in the pic is for the mystery Tremolo oscillator tube cathode bypass

When I had checked the bias supply, and all seemed good, I tried to bring the amp up slow on the variac and instantly got a odd hum out of the test speaker. I knew the only way this could happen in a tube amp is when the high voltage primary winding is shorting or leaking to the secondary. 

I checked to see if DC was on the speaker out by pulling the speaker off, and hooking up my Simpson VOM. Voltage instantly came up across the speaker out, as this amp has a diode rectifier. 

I then measured primary to secondary and there was a low ohms reading. The customer told me the amp never had any real power, and two techs had just checked it. 

The two "techs"? never bothered to actually unsolder one end of each bias cap to see if they were any good, or check the idle. I found the second cap was leaking bad, and the first was starting to go also.

Then, with low neg bias, the high idle current "smoked" the primary winding till it shorted across to the secondary, and then the "Hum"in the speaker.

I knew the Classic Tone transformers were really great sound, so I emailed them and was told they were just coming out with the new 2.2k primary for 100 watt amps.  This is a EL34 amp so I decided on the sweeter transformer as the amp has enough "grain" from the preamp. The transformer goes on fine, having to drill just two holes towards the choke for those mounting brackets. 

I "bias" El34s and 6L6gcs at 18 watts, and 6550s at 20 to 22 watts. I use the measure 
the voltage drop across the transformer primary method. 

Some important tasks on old British amps 

A. I make absolute sure the negative bias supply is solid and 
steady, before going any further, as you do not want to blow a original part

B. I "goop", a black rubber sealant, Permatex, from Kragens, that does not 
conduct, drys fast, and peels off easy, unlike silicon, over all the high voltage 
connections. The plate and screen pins, all HV power supply connections, standby 
switch, AC switch, and fuses. This makes the amp "quieter" and more stable, or less 
likely to blow up, and there is nothing to touch that will spark one of your pinkys! 

C. I clean-up everything, so that there is nothing that would short out, or leak high voltage to ground or other places not good for that to happen. I clean around the tube socket pins so the HV will not leak to the grid, and make sure all wires run decently and connections are solid and clean.

 This amp sounds super warm

I listened to the local smooth jazz station for a while and it sounded great, very Hi-Fi! 

This is a rare occurrence with a guitar amp, as most are way to bright and hot to actually listen to music with and to enjoy the smooth tone. The amp also comes up very easy, unlike the low-impedance transformers as there is some "cushion" for the output tube plates.

Super quality part, and super great service from ClassicTone company

Recommend ClassicTone Highly!