100w non-master kit keeps blowing tubes! HELP!

[d20]

Hey, since there isn't a dedicated NM 100w forum, I chose to place it here. If you know of a better place, please let me know and I'll move it.

I've attached some basic pics of the internals...if you'd like better, just let me know.

Ok...I'm blowing power tubes in the V4 and V5 positions left and right. The amp powers up fine, I bias the amp to well within tube tolerances (~33mA), and all my pertinent resistor values check out. Throw the standby to 'on' and the thing sounds epic...at low volumes. If I play the amp past 3-4 for very long 10-15 minutes I blow one or both of V4 and V5. Said blowage is sometimes, but not always accompanied by the H.T. fuse blowing. I've also noticed that twice the 1ohm resistor connecting pins 1 and 8 to the ground lug of V5 have small scorched areas (3-4mm) and some of the coating is blown off. Anyway, I've been through two quads of Tung-sols (luckily, the TubeDepot guys let me exchange them) and just today I've blown 3 of 8 SEDs that I received today.

Please help, myself and the TubeDepot guys are stumped...

[Roe]

have you checked for redplating or oscillation?

[Vegard]

That was a nice build!! Boards from Brian?

What is your voltages without the tubes?

[d20]

have you checked for redplating or oscillation?

I've watched the tubes for redplating and I didn't see any. I have not checked for oscillation and frankly, don't know how. Would you be able to talk me through that? FYI...I do have a oscilloscope, but don't know how to use it.

Thanks!

[d20]

That was a nice build!! Boards from Brian?

What is your voltages without the tubes?

Thanks! Yes indeed, board from Brian...that dude does some great work.

Here are my voltages with and without tubes.
With...
pin1: GND
pin2: 3.2VAC
pin3: 476VDC
pin4: 471VDC
pin5: -43VDC
pin6: 471VDC
pin7: 3.2VAC
pin8: GND

Without...
pin1: GND
pin2: 3.2VAC
pin3: 502VDC
pin4: 501VDC
pin5: -44VDC
pin6: 501VDC
pin7: 3.2VAC
pin8: GND

Thanks!

[Roe]

voltages look good. could you measure voltages under full load (at 10) and double check for redplating on v4-5?

[Roe]

sorry for the stupid question but have you watched for redplating when playing (at full volume)?

[d20]

voltages look good. could you measure voltages under full load (at 10) and double check for redplating on v4-5?

Sure thing.

When 'dimed', the voltages are...
pin1: GND
pin2: 3.2VAC
pin3: On V4 and V5, 477VDC (accompanied by a VERY loud sound that I can only describe as a low-frequency buzz/hum/whir/frappe/like someone fired up a spaceship/or fired up said spaceship's tunneling death ray). On V6 and V7, 482VDC (accompanied by what seemed to be the standard crackle/hiss/pop I got while measuring on pins 4 and 6).
pin4: 477VDC
pin5: -44VDC
pin6: 482VDC
pin7: 3.2VAC
pin8: GND

The thing's been running for a good 10 minutes and I see no evidence of redplating.

Thanks for the helps!

[d20]

sorry for the stupid question but have you watched for redplating when playing (at full volume)?

No stupid questions here. I've watched carefully and have never seen any evidence of redplating.

Thanks!

[Roe]

voltages look good. could you measure voltages under full load (at 10) and double check for redplating on v4-5?

Sure thing.

When 'dimed', the voltages are...
pin1: GND
pin2: 3.2VAC
pin3: On V4 and V5, 477VDC (accompanied by a VERY loud sound that I can only describe as a low-frequency buzz/hum/whir/frappe/like someone fired up a spaceship/or fired up said spaceship's tunneling death ray). On V6 and V7, 482VDC (accompanied by what seemed to be the standard crackle/hiss/pop I got while measuring on pins 4 and 6).
pin4: 477VDC
pin5: -44VDC
pin6: 482VDC
pin7: 3.2VAC
pin8: GND

The thing's been running for a good 10 minutes and I see no evidence of redplating.

Thanks for the helps!

this is strange. voltages should sag when running on 10. I guess I should have said more clearly that I meant running flat out. sorry.
So it doesn't even redplate when running flat out for some for some minutes?

[d20]

this is strange. voltages should sag when running on 10. I guess I should have said more clearly that I meant running flat out. sorry.
So it doesn't even redplate when running flat out for some for some minutes?

Ok...I'm just going to remeasure to make sure that I wasn't doing something wrong last time. I'll also be very explicit with any settings and tube configuration. So, once again...

With all tubes out and all controls set to 0:
pin1: GND
pin2: 3.5VAC
pin3: 506VDC
pin4: 505VDC
pin5: -44VDC
pin6: 505VDC
pin7: 3.5VAC
pin8: GND

With preamp tubes in, power tubes out, and all controls set to 0:
pin1: GND
pin2: 3.4VAC
pin3: 502VDC
pin4: 501VDC
pin5: -44VDC
pin6: 502VDC
pin7: 3.4VAC
pin8: GND

With all tubes in, tone controls dimed, and both volumes at 0:
pin1: GND
pin2: 3.3VAC
pin3: 480VDC
pin4: 473VDC
pin5: -44VDC
pin6: 478VDC
pin7: 3.3VAC
pin8: GND

With all tube in and all controls dimed:
pin1: GND
pin2: 3.3VAC
pin3: V4 & 5, 469VDC (LOUD hiss/whine/screech); V6 & 7, 480VDC (a single loud, low-frequency pop, then quiet). I'm not sure why my readings are so different less than an hour later.
pin4: 474VDC
pin5: -44VDC
pin6: 478VDC
pin7: 3.3VAC
pin8: GND

In every instance, I never noticed any evidence of redplating - which I have seen first hand while testing my JTM45 kit.

With the voltage drop on V4 & 5 (and the insane noise), there's something afoot there.

[Roe]

I mean running amp at full with signal through. so play the guitar or use a loop or something. and watch for redplating when the amp really works hard. also measure voltage drop. plates should drop to 430v or so. screens even lower

[d20]

I mean running amp at full with signal through. so play the guitar or use a loop or something. and watch for redplating when the amp really works hard. also measure voltage drop. plates should drop to 430v or so. screens even lower

Oh...

Ok...I ran an instrument level feed of AC/DC's Shoot to Thrill (YES!) into the amp. FYI...the readings at that level were done very hastily. Within thirty seconds, V5 started to redplate and blew before I could get all the measurements done.

pin1: GND
pin2: 3.3VAC
pin3: 420VDC
pin4: V4, 430VDC; V5, 375VDC; V6, 380VDC; V7, 380VDC
pin5: -44VDC
pin6: 365VDC...this is as far as I got...
pin7: 3.3VAC
pin8: GND

R.I.P. SED EL34...you were taken too soon...

You're tenacity is MUCH appreciated.

[Roe]

voltage readings look right (although v4 pin 4 is a little high). I assume your problem is redplating. please read the long thread about redplating I started recently (1959 w/larmar redplates).

a few tips that may remove the problem:
install a 100k on v3 grid
up the tail resistor to 100k or 47k
use 10k or 5k6 grid resistors on all output tubes
twist OT primaries and secondaries - and keep them apart (not even in parallel)
decrease the 82k to 68k

when the amp starts to redplate you need to turn down or even switch of

[d20]

voltage readings look right (although v4 pin 4 is a little high). I assume your problem is redplating. please read the long thread about redplating I started recently (1959 w/larmar redplates).

a few tips that may remove the problem:
install a 100k on v3 grid
up the tail resistor to 100k or 47k
use 10k or 5k6 grid resistors on all output tubes
twist OT primaries and secondaries - and keep them apart (not even in parallel)
decrease the 82k to 68k

when the amp starts to redplate you need to turn down or even switch of

Wow, that was an epic thread, I think I'll need to read it a few times for it all to sink in. Thanks!

- V3 grid...do you mean just lift the wires to pins 2 & 7, attached a pair of 100ks, and re-solder to pins 2 & 7? I'm assuming this is what you meant...
- What resistor is the 'tail' resistor? My wild guess would be the 10k that comes out of where the two 1M and the one 470 resistors converge. Correct?
- Per the instructions, I have 5k6 grid resistors on all the output tubes (you can sorta see them attached to pin5 on one of my pics)...I guess I'll have to raise this to 10k and see what happens.
- My OT primaries are twisted, but my secondaries are not AND I have them all running parallel AND zip-tied together per instructions...I'll mess with that although from just looking at it, I'm not sure where else I could put them.

Alright, I'll make some changes and reply back later.

THANKS AGAIN!