Super Lead late 10xxx series - weird voltages?

[Blues 1911]

Hello fellas, first post here.
I've built myself a Marshall 10xxx series replica, with Heyboer transformers (Drake 1203-80-ML PT with 240V primary and tap for reduced voltage (80%) on the secondary, and Drake 1202-132 OT) and Larry Grounding. Mustard coupling caps and Murata ceramics, just like in the old specimens. The bright cap wasn't still installed when the picture was taken, I actually decided for a historically correct 100pf, even if 5000pf sounded incredible and delightfully crunchy.



I've used the 80% taps and 8,2K drop resistor.

Voltages looks pretty much correct on the preamp, but are quite low on the power tubes despite my very high wall AC (245V!). It also happens that B+ on the rectifier output is around 420V in the beginning, and after some minutes (even 20) it suddenly falls around 400V:
______V1____V2____V3_____V4 and other power tubes
Pin 1__191___169___208
Pin 2_______________23
Pin 3_1.44____1___________400
Pin 4_____________________392
Pin 5
Pin 6__185___291___213
Pin 7________169____26
Pin 8__1.44___170___40
Pin 9

I've read that B+ on vintage 10xxxs was around 480V, so using the 80% taps should give even less than 400V indeed. What I'm wondering is if, with the lower voltage, preamp voltages shouldn't be lower respect to what I'm measuring. I fear something's not right somewhere.

I have used the 80% taps because I was expecting indeed a higher B+ (500V+). I have later discovered that 10xxxs seem to have a way lower B+, even while stock, respect to JTM 45/100s and JTM100s.
Using 80% taps looked also sensible in a prospective of tube preservation, as the EHX I have on aren't supposed to tolerate high voltages very well (especially on the EL34s screens).

I'd wish to know (better from someone who has measured voltages on an original 10xxx) if a 400V B+ is what to expect using the 80% taps on Drake 1203-80-ML power transformer, or if it is due to some kind of issue I have on my project. WIth a 460-480V stock B+, using the 80% taps should give even less than 400V indeed. What I'm also wondering is if, with the lower voltage, preamp voltages shouldn't be lower respect to what I'm measuring.

The questions are:
1) Is the 460/480V B+ reported from the old schematics to be assumed as correct for vintage 10xxxs?
2) With the lower voltage taps (80%), preamp voltages shouldn't be lower respect to what I'm measuring?
3 Shouldn't I be worried using EHX EL34s and 12AX7s with the stock voltages?

Thanks for your attention!

 

[Pete Farrington]

I've used the 80% taps and 8,2K drop resistor

My take is that the HT dropper is tweaked to get a good compromise between enabling facilitating a surfeit of output voltage swing from the V3 LTP if the value is low eg 8k2 (good for clean output but may cause a tendency to bias shift / blocking distortion as overdrive depth increases) and the opposite if the value is high, eg 20k.
Also the earlier Marshalls used beam pentodes eg KT66, which (compared to EL34) required higher bias voltage and hence a larger drive signal and hence a lower value HT dropper.

I hope the above may provide something background in regard of your second question.

It also happens that B+ on the rectifier output is around 420V in the beginning, and after some minutes (even 20) it suddenly falls around 400V

That seems weird, potentially concerning, I suggest to investigate that further.
eg it may indicate that the circuit current draw suddenly increases; what happens to the mV across the 1ohm cathode current sensing resistors on the output valves?

I can’t help with question 1, sorry, but if you want to try running the HT up at 500V, I suggest ‘valve stress mitigation’ of increasing the screen grid resistor value to 1k5 or 2k2, or adding resistance to the choke, to bring the total resistance between the reservoir and screen grid nodes to about 500ohms.
Also to elevate the heater circuit to about 80V DC and add a diode + resistor between the grid and cathode of the cathode follower section of V2, to prevent massive over voltage at power up that otherwise occurs, see http://www.valvewizard.co.uk/dccf.html

 

[XTRXTR]

^^^^^^
That is some great info from Pete right there.
With a Variac you could run your 240 taps at 240 and give everything a baseline bench test to compare all your voltages as well as hearing any changes that may occur otherwise. Of course start by dialing it to the same initial point you are at and then bring it to full 240 from there.

Raised heaters and V2 diode protection is always good.

That's a great looking build BTW well doneand welcome to the forum

 

[TAZIN]

Here's a few examples of the plate voltage on original Marshall's running the Drake 1203-80 PT.

500Vdc; A 'black flag' era with dual 10DB8 rectifiers. Filtering: eight 32uF caps and a 32 - 32uF preamp filter.

503Vdc; A mid-'67 JMP running a single bridge rectifier with filtering as follows: two 100uF cans for the mains, two 32 - 32uF cans for screens, single top side mounted 32uF cap for P.I., and a 32 - 32uF cap for the preamp.

509Vdc; A mid-'67 JMP running a single bridge rectifier with filtering as follows: four 100uF cans for the mains & screens, single top side mounted 32uF cap for P.I., and a 32 - 32uF cap for the preamp.

 

[Blues 1911]

That seems weird, potentially concerning, I suggest to investigate that further.
eg it may indicate that the circuit current draw suddenly increases; what happens to the mV across the 1ohm cathode current sensing resistors on the output valves?

Thank you for the extensive reply!
I don't know what happened to 1 ohm cathode resistors when the voltage fell from 420V to 400V, it just happened two times while I was measuring but I am lead to think it was a problem of severe wall AC drifting - during the day I read 245V, in the afternoon It often falls down to 235V. Truth is my house (and my amps) would get huge benefits from a good voltage stabilizer.
Lacking that, anyway, right in these days I've ended collecting parts for building at least the Geofex vintage voltage adapter with switchable -6V and -12V options

Here's a few examples of the plate voltage on original Marshall's running the Drake 1203-80 PT.

500Vdc; A 'black flag' era with dual 10DB8 rectifiers. Filtering: eight 32uF caps and a 32 - 32uF preamp filter.

503Vdc; A mid-'67 JMP running a single bridge rectifier with filtering as follows: two 100uF cans for the mains, two 32 - 32uF cans for screens, single top side mounted 32uF cap for P.I., and a 32 - 32uF cap for the preamp.

509Vdc; A mid-'67 JMP running a single bridge rectifier with filtering as follows: four 100uF cans for the mains & screens, single top side mounted 32uF cap for P.I., and a 32 - 32uF cap for the preamp.

Great! Thank you!
Seems that 400V voltage on the 80% taps is spot-on correct, then. I guess that I'd better keep the low voltage taps, as according to datasheet the EHX EL34 I have on my 10xxx can't stand more than 425V on the screens.

 

[Kats]

I built that amp as well, but in my research found the 5000pf cap was also a historically correct value in 1967. I had put a push/pull on the volume pot to switch between SB (no cap) and SL spec.