Super Lead late 10xxx series - weird voltages?

[Blues1911]

Hello,
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. Still no bright cap, but I'd probably go with a 4700pf even if not historically correct.

https://imgur.com/a/JlZ668q

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 specimens 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.

Thanks for your attention!

[danman]

Congratulations on your build! Two questions...are you plugged in to a lightbulb limiter when taking these measurements and what does the B+ look like without the power tubes installed?

[Blues1911]

Congratulations on your build! Two questions...are you plugged in to a lightbulb limiter when taking these measurements and what does the B+ look like without the power tubes installed?

Thank you! I've never used a lightbulb limiter (I don't even have one); about the voltages, without power tubes installed the B+ is something around 420-430V (20-30V more).
I've also tried with another EL34 set, with the same readings.
On the PT secondaries I have around 310VAC.

[danman]

I didn't catch the part about using the reduced voltage tap on the PT. I'd say your preamp numbers look about right for a single 8.2k dropping resistor with the B+ you have. I wouldn't worry about lowering them anymore or you will lose that classic Marshall sound. Lower voltages in the preamp will make the sound a bit "browner" with less of that classic Marshall "kerrang". A hotter bias on the power tubes will also bring the voltages down throughout the entire amp.

Most of the the original amps that had a B+ in the upper 400's were running either a 10k/10k or 8.2k/10k dropping resistor pair. They needed this much resistance because the B+ was so high. In the late 70's, Marshall started using a lower voltage PT in some of their 50 watt models. With a B+ in the upper 300's, Marshall chose to use a single 10k or single 8.2k dropping resistor to keep the preamp voltages in the correct range while running such a low B+ on the power tubes. For some reason Marshall only made this change to the 50 watt amps while leaving the 100 watters as they were. I only mentioned this change to the 50 watters to give you an idea of the values that Marshall chose to use when running a lower voltage PT. If you are set on lowering preamp voltages even more, you can swap your 8.2k for a 15k or 20k to see if you like the results. It will take the preamp voltages a bit lower than what Marshall originally intended but you may prefer the spongier, browner sound.

Can I ask why you chose to run such a low B+. Was it to bring down the volume or change the sound of the amp?

[Blues1911]

Can I ask why you chose to run such a low B+. Was it to bring down the volume or change the sound of the amp?

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 the 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 have asked about it exactly to understand (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.

Is the 460/480V B+ reported from the old schematics to be assumed as correct?

[Elad E]

Hey,
I built a similar amp earlier this year (I put up a thread about the build).
I'm using the regular taps and get about 460V (wall voltages range 232-236VAC) - all voltages check fine and the amp roars.
Personally, I wouldn't stress it too much, I'd set it up with the regular taps and 460-480VDC should deliver the goods and would be inline with all I've read about the original 10 series JMPs.

[danman]

As far as I'm aware, all of those 100 watt Superleads ran at least 450v or higher. I have never measured one personally though so I can't speak from experience. I believe that you will be fine at anything up to 480v or so as long as you are running with a conservative bias of 70% or less. If you hook up the regular taps and the voltage is a bit higher than you like, you can build a small and inexpensive "brown box" that will knock down the incoming wall voltage to 110-115vac. This will bring your B+ down into a slightly better zone.

Many of our members (neikeel, Tazin and a few others) hang out over at "The Marshall Forum" now. Those fellas have direct experience with many of these older circuits and could tell you exactly what they should be running at.
https://www.marshallforum.com/

[Blues1911]

I'd set it up with the regular taps and 460-480VDC should deliver the goods and would be inline with all I've read about the original 10 series JMPs.

As far as I'm aware, all of those 100 watt Superleads ran at least 450v or higher. I have never measured one personally though so I can't speak from experience. I believe that you will be fine at anything up to 480v or so as long as you are running with a conservative bias of 70% or less.

My only reserve on that is about EHX EL34s and 12AX7s early failure due to high voltages.

If you hook up the regular taps and the voltage is a bit higher than you like, you can build a small and inexpensive "brown box" that will knock down the incoming wall voltage to 110-115vac. This will bring your B+ down into a slightly better zone.

Right in these days I've ended collecting parts for building the Geofex vintage voltage adapter with switchable -6V and -12V options

http://www.geofex.com/article_folders/v ... ntvolt.htm

Many of our members (neikeel, Tazin and a few others) hang out over at "The Marshall Forum" now. Those fellas have direct experience with many of these older circuits and could tell you exactly what they should be running at.
https://www.marshallforum.com/

Done thank you!

[danman]

Glad to see that you joined up over their. You mentioned in your last post that you were also concerned about high voltages on the 12ax7 tubes. 12ax7's can easily handle 300v without a sweat so I see nothing in your voltage readings that would concern me. One half of the V2 tube is used as a cathode follower and some tubes can have a hard time dealing with that. Many Marshall owners complain about new production tubes failing early in the v2 spot because of the cathode follower. Using a nice vintage tube in this spot is one way of dealing with the issue as they seem to hold up much better than the newer stuff. There is also an easy mod that you can perform that will help protect and prolong the life of the tube in this spot. Either way, your voltages for the entire preamp look just fine and trying to run them any lower will change the sound of the amp.

As I mentioned earlier, you could build yourself a simple "brown box" voltage reducer that can mount inside of the head cab. This will allow you to run the full voltage taps but still keep the B+ in a safe range. El34's can easily handle 500v as long as the bias is set in a conservative range. Running the proper speaker load is also extremely important with Marshall circuits and many amps have been damaged by running at the wrong impedance.

One last question...you said that the B+ drops after 20 minutes. What B+ are you reading before you see it drop?

[Blues1911]

One half of the V2 tube is used as a cathode follower and some tubes can have a hard time dealing with that. Many Marshall owners complain about new production tubes failing early in the v2 spot because of the cathode follower. Using a nice vintage tube in this spot is one way of dealing with the issue as they seem to hold up much better than the newer stuff. There is also an easy mod that you can perform that will help protect and prolong the life of the tube in this spot. Either way, your voltages for the entire preamp look just fine and trying to run them any lower will change the sound of the amp.

I know, the only preamp failure I have registered in the lat 10 years was about a Tung-Sol RI 12AX7 on V2 in my JTM45 combo (1x12 Bluesbreaker)... even if I used conservative taps there, too! Anyway putting a NOS tube on V2 seems a bit of a waste, to me, especially considering that my voltages are easily in the datasheet tolerance range.

One last question...you said that the B+ drops after 20 minutes. What B+ are you reading before you see it drop?

The voltage fell from 420V to 400V, but given that it just happened two times while I was measuring, I am actually 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.

[danman]

The v2 (cathode follower) issue can pop up even when the preamp voltages are within spec. It has to do with the way the second triode in the v2 tube is wired that places it at risk. There is a diode mod that can be done that will help prevent the issue from occurring. I believe the fellas over at the Marshall Forum also mentioned that in one of their post. Of all of the new production tubes, the ones made in China seem to handle the v2 spot the best. Unfortunately that plant burnt down and hasn't restarted production yet I don't believe.