EL84's at 600v plate?

[Gene]

Hi,

I am servicing a Seymour Duncan 84-40 amp. It had a blown power filter cap and I replaced it. The amp works now but the plate voltage on the 4 EL84's are at 600v. If I recall, isn't the max on EL84's 300v or so? The cathode current is at 13ma. So the dissipation is in range although a little hot. I can see why the filter cap (220uf @ 350v) blew.

Here is the schematic:
http://www.schematicheaven.com/newam...ncan_84-40.pdf

Any one have experience with this amp?

Gene

[fullerplast]

Originally Posted by Gene

Hi,

I am servicing a Seymour Duncan 84-40 amp. It had a blown power filter cap and I replaced it. The amp works now but the plate voltage on the 4 EL84's are at 600v. If I recall, isn't the max on EL84's 300v or so? The cathode current is at 13ma. So the dissipation is in range although a little hot. I can see why the filter cap (220uf @ 350v) blew.

Here is the schematic:
http://www.schematicheaven.com/newam...ncan_84-40.pdf

Any one have experience with this amp?

Gene

It's not quite as bad as it seems for the filter caps. That amp uses a B+ doubler circuit so each of the two filter caps see only 300V across them. The plate voltage is high for EL84's, hence the low current to keep it within spec. It is possible that not all EL84's can handle it, but it seems just about everybody ignores spec on these poor tubes.

I'd suggest replacing the filter caps and go from there. It sounds like you are taking measurements with the tubes in place. If not, you'll need to do that for valid measurements. I suspect the amp is now running as designed.

[Gene]

Yes, I took measurements with the tubes in place. I accidentally killed my meter measuring plate voltage. I swear I thought I saw the meter set to voltage but alas, it was set to the notch right next to it for resistance. ouch... On the other hand, a good excuse to pick up the Fluke I've been eyeing.

I took a careful look at the PCB to see if any components spec might have been changed but did not see any. The amp actually sounds pretty good after the filter cap replacement.

I'll contact Duncan to see if this 84-40 amp was really designed to run at 600v plate just to confirm.

Thanks for your reply.

Gene

[phsyconoodler]

I would venture to say that it probably is.In order to get 40 watts out of 4 EL84's you have to have an insane plate voltgae.The current is what kills the tubes,so it is likely biased pretty low.

[Old Tele man]

...but, remember, while high PLATE voltage is a killer at IDLE, it's really the SCREEN grid voltage that controls/sets both plate CURRENT and OUTPUT power!

...and many european PENTODES were/are designed to operate with 500-600Vp but only with 300-350Vs, not like BEAM tubes where Vp = Vs = 450-500Vdc!

[Gene]

The schematic shows screen grid voltage of 275. The cathode current currently(sorry) measures 13 to 14ma so the plate current is a bit less. So yes, the tubes are dissipating in the correct range.

What's up with the design inside though? There are so many areas that can break apart (pots, the preamp tubes socket soldered to pcb, the input jacks with the plastic mounting screws, etc...)

[tremolux]

I think if SD had a chance to go back in time he might make some major changes in the build/design of both of his past amp models, the 84/40 and the Convertible line. I used to know where he had them built, I remember that some of the assembly took place in the states...but I think the circuit boards and most of the components were made somewhere in the pacific rim. Neither line was around for very long with short production runs, but they must have looked pretty good on paper. I worked on a Convertible that had been partially PtoP and had several upgrades... and the various tube driven preamp modules that plugged into the board sounded pretty good.

....also, just remembered some of the 84-40s came with Tungsram EL84s and ECC83s which were pretty decent tubes, somewhat along the lines of the older EI and Tesla in quality and sound...

[Gene]

The original tubes are long gone. It has EH EL84s and JJ ECC83s. The speaker is not original either. Do you know what the original speaker might be? It has single 12 inch. I think there was another model with 2 10s?

It was a pain looking for a radial 220uf/350v filter cap that actually fit the chassis. It seems all the ones I can find are 2inches tall. That is too tall to clear the chassis going into the cab.

[tremolux]

Hi Gene....there were two models as you mention and I recall someone talking about a few in head only cabs, tho Ive never seen one.

The speakers were out sourced from Eminence and had big gold stickers on the mag that said "King Tone Classic" and the ohm rating.

I hear you on the 220uf caps! I just recently found some 350vdc 220uf radial caps that barely fit an amp that originally had two 250vdc 220uf caps in series made by defunct company..... they were current production IC caps (not first choice, but ok for the amp they went in).

Theres an owners manual here...not real informative, but a few basics are covered. http://www.seymourduncan.com/images/84_40.pdf

[Gene]

I do have the manual pdf but thanks.
I had to improvise with the 220uf and tie it down horizontally. The customer really likes this amp though.