just realized, im not class A

[drewl]

okay, none of these are REAL class A, but while tinkering and doing research last night, with plate voltage of 430v, my 18w aint running what it's supposed to.
I lowered the input voltage w/a variac to bring it down to what the schems read, but it sounded the same.
I love the way this amp sounds, but should I drop my voltage internally to get closer to what it should be?
right now it's running around 80ma of bias with 16v across a 200ohm cathode resistor, but the paltes aren't red.

[zaphod_phil]

Yes, all the 18watters are genuine Class AB amps, or you would never get 18W out of them!

Having said that, there's no way you should be putting 430V onto any EL84s. That's way past their max voltage ratings, and when the amp is running the plate voltage will actually swing above and below B+. If you're using 7189As (kind of military super EL84s), they may be able to handle the voltage.

Power dissipation wise, if we allow for 10mA total going through the screen grids, and the cathode to anode volatge is 414 volts, that's a dissipation of 414 * 0.07 / 2 = 14.5W per tube, where the max dissipation is 12W. I hope those are miltart-spec tubes! In Class AB they should be biased to around 70% of max dissipation. So while your EL84s may not be redplating at the moment, they're going to have a pretty short lifetime in your amp!

In short, there's a serious problem there that needs fixing. Using a variac also isn't the right way to fix it, as if you reduce the heater voltage to less than 10% of 6.3V, you run the risk of stripping the cathodes.

[drewl]

Funny you mentioned 7189's...that's what tubes the trannies were used with in a Gibson amp.....
I'll drop the voltage properly to avoid melting my EL84's!

[zaphod_phil]

Most likely they were actually 7189As, which are rated up to 440V and 13W dissipation. 7189s are only rated for 400V and 12W dissipation. You can get EL84Ms that are the same rating as 7189s, but not everyone likes their tone.

[Ethan]

I have a '61 Tremolux that runs a pair of EL84s at 440V and I've not had problems running NOS GE 6BQ5s. Fender ran these tubes very hot at 420V and since mains voltage has gone up since '61... 440V.
So far so good, but I am planning to drop it a little just because it makes me nervous.
Perhaps that's why Fender only used these tubes in the Tremolux for less than a year!

Peace-
Ethan

[zaphod_phil]

I think you've been lucky. Regular EL84s aren't that robust and shouldn't be run at those voltages. Now what kind of a Tremolux are saying you have? All the ones I've ever come across (5E9, 5G9,6G9,AA763, AB763 ) either had 6V6s or 6L6s in them - not EL84/6BQ5s. And are you sure that the amp wasn't originally designed to take 7189As?

[Ethan]

6G9A. It's legit. They went from 6V6s to 6BQ5s briefly in '61, then thought better of it and switched to 6L6s. As far as I know it's the only old Fender amp that uses this tube, and yes, the tubes are running at much too high a voltage. The schematic actually lists 400V (still way too high, but Fender was notorious for exceeding plate voltages) and if you add the POM 20% design factor and 40 years of increasing mains voltage, it's not that far off.
I've been running those tubes at around 19mA and have had no trouble and no glow on the plates. I am nervous about it, though and plan to put in a 5V4 rectifier to drop it a bit and re-bias, but haven't had the time.
Incidentally, the amp sounds like a million bucks!
I don't know whether Fender switched to 6L6s to cater to the ever increasing demand for output power in the early sixties, or whether they thought better of the voltages, but it was only about a 9-month run before they introduced the 6G9B with 6L6s.

Peace,
Ethan

[zaphod_phil]

How long have you had the same tubes worked in that amp? I suspect they would have had a short but glorious-sounding life in the Tremolux! As Kevin O Conner says, EL84s are soft-vaccuum tubes and don't handle over-stress the way 6V6s and 6L6s can.

[Ethan]

Over a year, but I don't use it often. There were Groove Tubes in it when I bought it that start to glow red much easier. The GEs seem to be pretty tough little tubes. Lucky, I guess. I agree with you whole-heartedly, though. 440V is slamming the cr*p out of 'em and I'm surprised (pleasantly) that they have such a sense of humor about it. The Groove Tubes didn't.
All the same, it'll be worth my while to drop the voltage a bit (or even a lot) when I find time, otherwise I'm sure to let the smoke out of that amp eventually!
E-

[zaphod_phil]

Well, it would be a shame to wear out those nice NOS GEs before their time! If they were some cheapo Chinese ones, that might be a different matter....

[Ethan]

Right on. Let 'em burn.

[DeanOwens]

6G9A. It's legit. They went from 6V6s to 6BQ5s briefly in '61, then thought better of it and switched to 6L6s. As far as I know it's the only old Fender amp that uses this tube...
Incidentally, the amp sounds like a million bucks!
I don't know whether Fender switched to 6L6s to cater to the ever increasing demand for output power in the early sixties, or whether they thought better of the voltages, but it was only about a 9-month run before they introduced the 6G9B with 6L6s.

Peace,
Ethan

i would love to come accross one of these. does it sound like a fender? i've almost been tempted to build one just to check it out. i don't think i've ever seen one for sale.

and as to why they changed - leo was always on the quest for a clean amp. he didn't like the dirt. he also was someone who tinkered a lot. i remember reading where he had to be called to the office because he kept changing values and it was making it difficult for the guys to give the amps a once over before it left the plant. he wasn't too happy, after all, it was his company.

anyway, it was probably him just trying something new. even after they switched to the 6L6's, the tremolux didn't stay around very long. they basically had it covered with the vibrolux and the vibrolux reverb.

i know it's an 18watt board, but if you could post some sound samples somewher eand let me know i would apreciate it. i would like to hear one of these.

[drewl]

well i lowered my B+ with a 1.3k 10w resistor for now, plate v is around 250, rebiased and she sounds real sweet...more marshall grind. Only problem is power is down to about 10-12watts...so i thinks me is gonna raise B+ just a bit.
Anyone have a link to lowering B+ using zeners?
...I know I've seen them around.
one other thing, it seems with just my guitar I'm not driving the input as much as I can....(sensitivity) using a boost really kicks up the power on this baby...this normal?
thanks all....just trying to get it right...not bad considering I used old junk parts that were headed for the dumpster!

[Ethan]

Dean,

Sorry I took so long to answer. Had trouble posting. All better now.

The Tremolux does indeed sound like a Fender. Lots of clean headroom and that classic blackface sound. It does have a slightly different breakup than my 6V6 Fenders; a little brighter, "chimier" perhaps, but I wouldn't really call it a British sound at all. It's undoubtedly Fender all the way.
I really doubt Leo was "trying something new". I suspect it's more likely he got a big lot of 6BQ5s for cheap and used them up on the Tremoluxes. He was, after all, notorious for buying surplus. His "mistake" is my gain, however. I love the amp.
As for building a clone, I say go for it. The layout is pretty straightforeward (typical blackface) and it's a great sounding amp. If I were doing it, I might opt for a lower voltage power trannie. As mentioned earlier in this post, there's no sense in battering your power tubes for no good reason, and the stock circuit runs them WAY too hot. I might try changing the dropping resistors in the power supply to keep the stock voltages in the preamp and PI and only drop the power tube voltages. Might be a fun project.
If my memory serves, The tremolo affects both channels, which are basically the same. That means you could ditch a channel altogether for simplicity's sake or... ...modify? Oh, man! Who's gonna host the 6G9A Tremolux site!?! Marshalux, anyone?

Happy soldering-
Ethan

[AmishWarlord]

On the zener subject, if memory serves, just put them in the CT, facing backwards. so.. something like this

-----(ct)----|<-----|<-----gnd

if that made any sense. be sure to use more than one, so the power is dissipated over all of them. If you're trying to drop 120V, use 5 24V zeners. When they blow, then tend to go short circuit, so that way you'll only have 24 extra volts all of a sudden instead of 120. Someone correct me if i'm wrong anywhere in here... i'm still in the planning stages of my first amp, so, i may not be the most trusted resource here.

-J