High voltages on preamp tubes (pins 1&6)

[Enrico]

Hello!

I've completed my '67 100W (I've used the 240V tap on the PT for EU
230V, which usually is 232-233V here in Rome); everything seems right
but...

When measuring voltages on the preamp tubes (without the EL 34
installed) I have more than +20% on pin1 of V1 and more than +20% on
pin6 of both V2 and V3 (especially V2 which measures a huge 335V !!!). Swapping tubes changes this a
bit but not in a radical way.

Voltages on the output tubes sockets are within 5%, though... and the
heater's AC is 3,33 instead of 3,15 (also without the output tubes) but
it seems fine (within 5%)...

I will post some photos of the board soon), but the wiring seems alright with right resistors, well-oriented caps and
so on, so...

What should I check???

Thanks,
Enrico.

[Billy Batz]

I wouldnt be worried. The voltage chart your using may be for an amp with 2 decoupling resistors between the screens and V3- the point at which your voltages are higher- and thats the difference there between 8.2k and 20k which even at idle is a lot. Maybe even 20%. If you have a single 8.2k resistor in taht position (near the bias pot) then using 2 10k's as later amps would would more then double the voltage drop you have between the screens and V3 which could already be near 80+v with a single 8.2k. The few extra volts AC from your house will raise voltages in the amp a bit, and putting in the EL34s will drop them a bit. My 67 clone's voltages arent far off from yours. I have schematics with voltages of early amps with 560+ B+ who had 390v on V1 and V2

[Enrico]

Thanks for your reply!

Just a few doubts:
1) Isn't the max rating for those 12AX7 just 300V (as I've been told by mail by the technical staff @ metroamp)?

2) I've added the second resistor just to try it (a 10K ,2W along with the 8,2K as shown in some schematics of '67 amps) and the voltages dropped a bit: 205V instead of 220 on V1pin 1, 310V instead of 330V on V2, pin 6, but now it blows the mains fuse from time to time, which didn't happen even with the EL34 installed... so with an even higher current draw.
Is it normal to blow more fuses without the output tubes installed?

Thanks,
Enrico.

[PCollen]

Hello!

I've completed my '67 100W (I've used the 240V tap on the PT for EU
230V, which usually is 232-233V here in Rome); everything seems right
but...

When measuring voltages on the preamp tubes (without the EL 34
installed) I have more than +20% on pin1 of V1 and more than +20% on
pin6 of both V2 and V3 (especially V2 which measures a huge 335V !!!). Swapping tubes changes this a
bit but not in a radical way.

Voltages on the output tubes sockets are within 5%, though... and the
heater's AC is 3,33 instead of 3,15 (also without the output tubes) but
it seems fine (within 5%)...

I will post some photos of the board soon), but the wiring seems alright with right resistors, well-oriented caps and
so on, so...

What should I check???

Thanks,
Enrico.

Check a) your supply voltage values to the tubes, b) the voltage drops across the plate resistors , c) the voltage value on the plates of the tubes themselves, and the cathode voltages of each tube.

When the power tubes are installed and are properly biased, you will see a significant reduction in voltages throughout the amp. The 12AX7's draw only a few mA combined at quiescence, while the power tubes can draw 35 to 45mA EACH depending upon plate voltage and bias point.

[Enrico]

Thanks for your replies guys!!!

Everything seems ok except for the plates voltages without the output tubes.
When I installed te EL34 the voltages dropped significantly:
for instance pin6 of V6 dropped from 335V to 280V...

I checked the bias once again (all 4 power tubes are within 33 and 38mA) and played through the amp a bit during reharsals... Great (but a little too dark) sound, great dynamics, but the amp produces some "cracks" during the most powerful single note playing... UHM Confused.

PT and OT seem quite cold and nice.

The problem is it blows the mais fuse (never blows the H.T.) too often when I power it up and/or when I switch from standby to "on" after a break... and yes, I'm using 3,15A slo blow mains fuse as specified in all Marshall 100W reissues in Europe (230V)...

Hope someone can help!!!

[Enrico]

Anybody???

Actually, I've got some news: the mains fuse wasn't blown, the amp just turned itself off as I moved from standby to "on" after a short break...
I'm having some troubles with that mains fuseholder: good fuses that didn't let current flow due to fuseholder malfunctioning (could bad contacts have been the cause of blown fuses???) and so on...

Anyways, the crackling problem during hard picking on medium-bass frequencies still remains: could I have used too low screen filtering (it is a F&T 16+16 for a total of 32uF)?. Or what could it be?

Thanks to anyone who will chime in!

Enrico.

[bluze81]

Enrico, as far as the crackling sounds your hearing, I suggest you go thru the amp with a meter check all your contacts, you do have good tubes in it? you could have a cold solder joint thats not contacting well, hopefully its an easy fix.

[Enrico]

Thanks, Bluze!

Tubes (JJE34L) should be fine, I just received them from Metroamps and they seem to bias correctly...
I'll check for cold solder joints but I guess that the sounds I've heard are not bad joints... Those cracklings were not as loud as the note they should have been: it seems like the amp can't stand those high volume passages... I can't explain it better, sorry!
Then I'm wondering if bad solder joints could cause fuse failure...
I hope so...
Enrico.

[neikeel]

Sounds like you have some oscillation related to the output section.

Go over the soldering on the pins of your output valves. If that does not work try another set of out put valves. Are your filter caps all adequately grounded?

If no joy you will need some pics for guys like Flames, SDM et al to help

[Enrico]

Hello everybody!!!
First of all, thanks for the replies.
I've changed my house so I've been out of the web for a while...
In the meantime I have changed my BB RI PT just to have a working amp to gig with, then I switched over to my '67 plexi clone build and discovered that:

- I had one of ther two mains (100uf) caps leaky: changed it and now the fuse doesn't blow (even the 2A slow blow recommended by George for EU).

- My jj E34L are ok (tested in a beautiful british navy tube tester).

- The problem of the too high preamp voltages is still there: even though I've measured lower voltages with all tubes installed and biased, as stated in a previous post, at 280v this monster managed to burn 2 old but good 12AX7 in V2 (a jj and an old chinese from 1990): I've added the second resistor as in later ('68) amps, but this seems to be insufficient, checked all the other values and they seem correct...
Any ideas???

Enrico.