High Plate voltage on 6L6GC

[danotron]

I am building an amp with a donated traynor PT. I am getting a plate voltage of 530Volts. I am assuming that this will kill 6L6GC's. Is there a meathod of lowering the voltage to 450? Should I switch to EL34's? If so the screen can only take 425v....how would I lower the voltage before it hit the screen?

[David Root]

NOS 7027As and 7581As will handle that but they are big bucks these days. You would have to drop the screen voltage some though I would think. You could use a 5W Zener diode string to ground to drop and hold the screen voltage.

I have seen something here about JJ 6L6GCs being able to handle Music Man PT plate voltages (680-700V) IIRC and I wouldn't be surprised if the Winged C Svetlanas could take 530V too.

Alternatively you could put a 50V to 80V 25W Zener between the PT CT and ground.

[tubeswell]

Alternatively you could put a 50V to 80V 25W Zener between the PT CT and ground.

Or a string of 5W 9V-15V zeners (say 6 x 5W 15V zeners in series) - with the banded (cathode) ends pointing to ground. Each one doesn't need to be huge this way, and you get to play around with tailoring the voltage dropped a bit more easily.

[PaisleyTube]

I am building an amp with a donated traynor PT. I am getting a plate voltage of 530Volts. I am assuming that this will kill 6L6GC's. Is there a meathod of lowering the voltage to 450? Should I switch to EL34's? If so the screen can only take 425v....how would I lower the voltage before it hit the screen?

Modern JJ 6L6s or JJ EL34s will survive.
(EL34s do also work in early 70s Marshall 100 Watt amps with about 520 Volts to the plates and screengrids)

Using zenerdiodes is an option but not that great, it does work ok in low powered amps (<20 Watt) but it's not the best option imho.

EDIT: if it does have a centretapped HT; you can also try using a Weber coppercap, you can choose from various kinds of sag and about 50 Volts (max) voltage-drop. quality does vary but this is a pretty easy option, you can get them as octal tubes or a s metal blocks.

[stoo]

If it's a old Traynor PT it could have been wound for 110V primarys. Check your filament voltage and see if it is high too. There is a way of dropping the 120vac to something closer to 110 using a small tranny in series with the incoming ac.
Stew

[jaysg]

It's my understanding (not experience) that old Ampeg V series amps can be tubed with =C= and other 6L6GCs. They run higher B+ than that. It's mostly about the screens and bias point....which in turn affect heat...so search on Ampeg info like www.ampegv4.com.

[Structo]

Curious about if the plate voltage you measured was with the amp fully loaded with tubes or not?

Depending how stiff the PT is, the added load of all the tubes may reduce that but probably not enough to be effective.

Did you happen to measure the screen voltage as well?

I'm mentioned this before but I made an amp from a Hammond organ amp.
It had 465v on the B+ with a 5U4GB rectifier.
The amp has vintage Sylvania 6V6GT tubes.

I installed a 125 v 25 watt Zener stud mounted through the floor of the chassis.
I also bypassed the zener with a .01uf cap to kill any noise.
The center tap of the HT winding was attached to the zener anode.

It dropped the voltage down around 340v.

It worked great but I got to tell you that that diode gets very hot!
I did use heat sink compound on it as well.
This zener has the cathode on the stud.

[Wayne]

I just had a Sovtek Mig-50 on my bench a couple of weeks ago. It was putting 560V on the plates and just a few volts less on the screens of it's 6L6s with no fireworks. The tubes that were in it were Fender branded, and I replaced them with customer-provided EHs - still no problem.

FWIW

W

[tubeswell]

I made an amp from a Hammond organ amp.
It had 465v on the B+ with a 5U4GB rectifier.
The amp has vintage Sylvania 6V6GT tubes.

I installed a 125 v 25 watt Zener stud mounted through the floor of the chassis.
I also bypassed the zener with a .01uf cap to kill any noise.
The center tap of the HT winding was attached to the zener anode.

It dropped the voltage down around 340v.

It worked great but I got to tell you that that diode gets very hot!
I did use heat sink compound on it as well.
This zener has the cathode on the stud.

I'm not surprized - you are dropping 125V so with the total peak current draw at around say 140mA, that could be 17.5W.

[Structo]

Do you think I am too close to the design limitations of that Zener?

I can't remember if they had any larger ones like 50 watts.

[j-po]

Using zenerdiodes is an option but not that great, it does work ok in low powered amps (<20 Watt) but it's not the best option imho.

Why not?

[JD0x0]

Take the (IMHO) better route and just switch to KT88 tubes, they last longer than 6L6's (usually) they can handle the voltages on the plates easily, and they sound great. better bass response than 6L6's and its not loose flubby bass like a 6L6's it's smooth tight and clean.

[Jana]

Only switch to Kt88's if the PT can handle the extra current required.

[danotron]

KT88's...great idea.. ..I will have to check and see if my PT and OT can handle it.....

[greiswig]

Take the (IMHO) better route and just switch to KT88 tubes, they last longer than 6L6's (usually) they can handle the voltages on the plates easily, and they sound great. better bass response than 6L6's and its not loose flubby bass like a 6L6's it's smooth tight and clean.

I'm using a pair of KT88s in my D'Lite build. I like them a lot. I did have one Winged C go south on me with only 100 hours or so on it, though. Currently using a pair of Pentas that sound great.