more headroom possible?

[Helmholtz]

Why would anyone who doesn't own a scope care ? It won't blow up immediately.

A scope is of no use here.
If you draw more than rated power from a transformer it will overheat and eventually short - except if the thermal fuse blows first.

[glebert]

Since the EL-84 is above the nominal primary impedance you want the speaker to be higher than the tap rating as well. I think if you went to an 8 ohm speaker you could try out the 4 or 8 ohm tap and see what sounds better. Unfortunately there aren't many 5 or 10 ohm guitar speakers which might be "best". Then again, as Enzo always said, it's just a guitar amp.

[Helmholtz]

Since the EL-84 is above the nominal primary impedance you want the speaker to be higher than the tap rating as well. I think if you went to an 8 ohm speaker you could try out the 4 or 8 ohm tap and see what sounds better.

As said, EL84s prefer 8k.
Connecting an 8 Ohm speaker to the 4 Ohm tap would result in a primary impedance of 2x6.6k = 13.2k, meaning low output and excessive screen dissipation.

[daz]

As said, EL84s prefer 8k.
Connecting an 8 Ohm speaker to the 4 Ohm tap would result in a primary impedance of 2x6.6k = 13.2k, meaning low output and excessive screen dissipation.

Thats why i asked tho. As i said, what would be the best speaker ohm rating and with which OT tap to be "as optimal as possible" with the trannys it has. Not asking what tap to use with the 4 ohm speaker. I'm saying i'm willing to get another speaker if there is a optimal combination of speaker ohm rating and OT tap with the tranny in it now that wouldn't be a big compromise in tone and reliability. If not, thats fine, i'd just pass then. I pass may anyways because now i'm seeing that el84s can't reliably handle 400v on the plates and it's around 395v. But from what u said sounds like a 4 ohm on the 4 ohm tap or any speaker rating on it's matching tap should be fine because 6.6k isn't a huge difference from 8k is it? Honest question, i don't know but from what i have read on this subject in the past it seems not.

[Helmholtz]

But from what u said sounds like a 4 ohm on the 4 ohm tap or any speaker rating on it's matching tap should be fine because 6.6k isn't a huge difference from 8k is it? .

Yes, that's the best match and will give max. power though not more than with 6V6s.
Nevertheless anything below 8k increases plate dissipation and is likely to reduce EL84 tube life especially with your B+.

[daz]

Yes, that's the best match and will give max. power though not more than with 6V6s.
Nevertheless anything below 8k increases plate dissipation and is likely to reduce EL84 tube life especially with your B+.

I measured the B+ after i wrote that and it's at 365 with the cathode bias. It was 395v when i have 6v6 and fixed bias so will it go back to 395v if i use fixed bias, and any way to drop it w/o affecting the preamp voltages? Parallel nodes and zeners on the power node?

[Helmholtz]

I measured the B+ after i wrote that and it's at 365 with the cathode bias. It was 395v when i have 6v6 and fixed bias so will it go back to 395v if i use fixed bias, and any way to drop it w/o affecting the preamp voltages?

It's not the cathode bias, but higher idle currents that lower B+.
BTW, cathode bias is safer with EL84s as it means lower plate-to-cathode voltage.

[daz]

It's not the cathode bias, but higher idle currents that lower B+.
BTW, cathode bias is safer with EL84s as it means lower plate-to-cathode voltage.

Isn't fixed supposed to be a bit louder and tighter tho? seems like it from my A/B experience with fixed vs CB in the past. (also has a fixed/CB switch on one amp) By the way, what screen values would u suggest?

[Helmholtz]

Isn't fixed supposed to be a bit louder and tighter tho?

Yes, fixed bias may result in 3W more output, but at the cost of tube life.
18W from a pair of EL84s is possible with cathode bias.and means pushing the tubes to their limit.

Dunno how to measure tightness.

[daz]

Yes, fixed bias may result in 3W more output, but at the cost of tube life.
18W from a pair of EL84s is possible with cathode bias.and means pushing the tubes to their limit.

Dunno how to measure tightness.

Well, it's an ear/feel thing. It's there because like i said i've had a switch in one amp so i was able to A/B them instantly. I stopped using CB long ago because i just like fixed better. CB is looser and fuzzier. Not a huge thing but i do like fixed better. I at least have to try it and see but so far even copying some FB schematics i found i can't get any negative range. It's odd. Not sure what i'm doing wrong but i'll figure it out eventually.

[pdf64]

Regarding the problem with a fixed bias supply, is the HT supply from a full wave bridge, or a 2 phase push pull rectifier?

I can’t help wondering why an EVM12L would need a new cab making?

[daz]

Regarding the problem with a fixed bias supply, is the HT supply from a full wave bridge, or a 2 phase push pull rectifier?

I can’t help wondering why an EVM12L would need a new cab making?

I guess the latter, tho i'm only familiar with the term "full wave bridge" which it isn't. As to the 12L, 1-i'm old and the thing is already on the verge of too heavy, but the big one is 2-it won't fit in the cabinet. Even the celestion 100 almost won't. It's a old marshall solid state 1x12 combo cab i built this tube amp into and being solid state they sized it small since theres no necessity to make room for trannys. I had to make a new baffleboard to make a speaker fit, and even then i have to make sure i loosen the speaker and let it fall to gain the maybe 1/16" play in the holes otherwise a tranny won't let me put the chassis in. 12L? Not till pigs fly or i make a new cabinet which is less likely than pigs flying.

[Chuck H]

The problem could be the 1K screen resistors are too big for 6V6. Be done with it and shove some EL34s in there.

I kinda thought the same thing. Daz is already concerned about the current capacity of the PT but it shouldn't matter what tube is used. Like when people arbitrarily put 6L6's in their Deluxe/Reverb and wonder why it's not louder than the 6V6's.

[Chuck H]

FWIW I get really good results using el84's at 365Vp and an efficient speaker (V30 in my own combo). I'm using a sort of cathode bias with a diode clamp on cathode voltage rise right after the onset of clipping. I'm also using a 160R cathode resistor and dissipating 11+W per tube at idle. So a 130R resistor at this Vp is probably drawing excessive current. AND...

I've also had good (but different tonal) results using el84's in fixed bias with the same PT. Because the tubes are biased cooler in fixed bias the plate voltage goes up to the 380 range but I've never had any trouble with tubes because of it. Some Peavey's, Mesa's, Traynor's, etc. run el84's at 400-ish Vp. I've owned a couple at one time or another and never had a problem with tubes. I mean, they don't last for a year or more like more conservatively operated tubes can, more like four to six months. But IMHE they don't fail abruptly or blow up. Run at higher voltages with fixed bias el84's take on a different character from the smooth/chimey VOX thing they're famous for. They really seem to grow a pair and deliver some impressive power. My usual experience with el84's in combo amps is that they become microphonic even before you can wear them out anyway. They're especially bad about this. Since I love this tube I've always taken the motto "Smoke 'em if you got 'em." Fortunately they're still the most affordable tube type.

[tedmich]

I used 12L's for many years. My fav speaker. Unfortunately that would cost me a lot of $ and work because i would have to buy a 12L AND all the materials to build a new cab then slave over it for god knows how long. I hate making cabinets. All that and no guarantees....no thanks ! Like i said above, if theres no way to get a bit more with the amp as is i'll pass and it'll go back into the garage.

some prefer this Eminence version anyway and the price is right (I bought one) https://www.ebay.com/itm/12490339636...4AAOSwc1hgwQgC