Marshall 1956 Super Lead

[deeaa]

Thx, yeah I can't have it in my livingroom once the family comes home from a trip so need to haul it back...and then I will only play it maybe twice a week at best.

Oh well, still have tonight to rock it somehow my regular home noodling setup doesn't quite entice me to play now.. ..

Sent from my SM-A125F using Tapatalk

[andrewsrea]

@deeaa : that sounds killer!

I find the post-inverter MV is worlds more useful than the pre-inverter MV.

I have an amp I built from a 80w tube Earth PA (4 - EL84), which has 3 channels: Fender BF Twin, JCM600 & Plexi (1959 Super Lead). Like your amp, the JCM600 channel has the pre-inverter and post-inverter master volumes. It really adds a lot of flexibility.

[tlarson58]

Great pics, videos and descriptions, @deeaa !

1.) The mystery jack appears to be a factory 'Direct Out,' for the purpose of recording or slaving other amps. This type takes the signal from the output transformer, rather than along the preamp chain as other designs do. It is not for additional speakers or headphones.

2.) The Master Volume mod looks copasetic and is the same type found in the JCM800 2203 model. It has been inserted between the Treble variable lug #2 and the blocking cap, just before the phase inverter tube #3. The reason this amp does not sound like a 2203, is the 2203 adds one cascading preamp, has more gain in the initial gain stage (higher plate resistor) and has a bias resistor in the 2nd stage that offsets the 'zero line' which compresses and clips asymmetrically. FYI: I prefer the Ken Fischer MV, which places a dual 1M pot MV between the inverter tube and the output tubes. A much more complicated install, though.

3.) The trimmer (variable resistor) pictured behind the Bass pot, is the bias control. It has been a while, but I believe fully clockwise = overbiased and counter clockwise reduces the bias, increasing the plate current and idle wattage. You can check if I have my direction correct with a digital volt meter (DVM). Have the DVM in a DC mode to measure a range of 90 volts or less (range will be 30VDC to 65VDC). Don't choose too high or you'll lose the detail of tenths of volts. Connect the black wire to ground (chassis is fine). Mark your trimmer position with a grease pencil or a dab of light color paint. With the amp plugged into a speaker, 'on' and in stand-by mode, apply the red probe to pin #5 (control grid) of an easy to access output tube. Write down the result, which will be a negative voltage. Using a plastic screwdriver, turn the trimmer a hair clockwise and recheck the voltage on pin #5 of the output tube. Compare to the first record and if it got more negative - I was right. If it got less negative, then counter-clockwise is the underbias position. I'll take you through biasing below.

4.) IMHO, @mozz could be right that 6CA7 could be identical to a brand's EL-34. There are some brands which are not and land in between a 6L6 and EL-34 as far as bias requirements. I know for sure that current production EHX 6CA7, 70's Sylvania 6CA7 and some GE 6CA7 (sometimes branded as RCA or Phillips) are not drop-in replacements for EL-34. The tell is typically a skinny EL-34 looking 6CA7 is equivalent and the 6CA7s with the tubby glass, are not. A true EL-34 is a 'kinkless-pentode' design and some 6CA7s are 'beam-tetrode' design. Many 6CA7s of this design can go higher than the max plate wattage of an EL-34, but I ignore this and treat it as extra reliability.

Bias: I suspect you have a Bias King type of device, to which you place in between an output tube socket and the actual tube. So I will give you my process accordingly:

Warning: amps have voltage with can kill you. Please proceed carefully. Be scripted with your moves in the amp.

1.) Calculate the wattage max and target. The Bias King tool inserts a small value, accurate resistor in series with the tube's cathode and ground. It measures the voltage drop across the resistor and does the math for the display, Which is the total tube current (plate+ screen).
It is safe to use 30w per tube for 6CA7as the max wattage (25w plate + 5w screen).
Max idle wattage = 21w (70% of max)
Target idle wattage = between 17w and 21w. The former is enough to mitigate bad sounding crossover distortion. As you apply less bias (running 'hotter'), the amp may get louder and less sterile (changes in the midrange & compression).
Wattage is calculated as: Plate voltage (pin #3) x milliamp reading from bias tool / 1000. Example: 405VDC x 55ma /1000 = 22.3w
note: plate voltage will decrease as you remove negative bias voltage, while increasing tube current. So, you have to adjust both the current and voltage in your calculation, each time you adjust.
2.) Connect the bias device and connect the black probe of your DVM to ground, with the DVM range set to 500v or less.
3.) Warm the amp up on standby, all knobs to zero and a speaker or 100w load plugged in, with the proper ohms selected for that load.
4.) Begin fully over-biased, put the amp in play mode (remove standby), record the plate voltage (red DVM probe to pin #3 of the output tube) and milliamps from the bias tool. Calculate the tube wattage.
5.) Reduce the bias by making a small turn on the trimmer. Watch for the tubes' grey plate getting an oval red glow in the middle of them - increase the bias if you see this. Repeat the notating and calculation.
6.) When you get to 17w, stop and play the amp. Play with the settings and listen. If you love the tone - stop. If not, set the controls to zero, take it to 18w and repeat. Caution: your ears like loud and early on, it may be the same tone - louder. So, listen critically.

I rarely go above 65% of max wattage with my amps. I've had 70% sound different, but great and then hours later it sounds like the tubes went dull.

I realize this is long and I may have distracted from your post, so my apologies and hope you found this useful.

You are such a good resource to have here.

[andrewsrea]

You are such a good resource to have here.

Thanks, Tommy! It is nice to be appreciated. Even nicer to share what I have learned over the years.