Contemplating a PT change or not?

[jojokeo]

I am wondering if a P-tran change would make much if any difference in a SE EL34/6L6-5881/KT77 amp I built. Common sense says to leave it alone and don't screw w/ things that are working and sounding great. BUT... When I switch to the various tubes there really isn't much if any perceiveable change in output volume. I have a tranny rated 330v @ 120mA to use if wanted but the changeout wouldn't be the easiest thing to get done as things are compact as it is. But a little extra voltage and capacity could get a bit more power? And the PT would be running a bit cooler negating the possible need for a fan?

The PT I used is rated 260v @ 100mA and when I've got the el34 in there w/ a proper Pdiss bias it's right at the 100mA rating and the voltage drops to exactly the 250v that the tube spec sheet says should produce 11watts. I'm unsure if it's actually putting out that much because when a 6l6 or 5881 is in there it sounds about the same loudness and those should only yield about 61/2 watts. Plus the el34 & kt77 get the tranny pretty hot to the touch after cranking it up for a good 1/2hour or more and the tranny is at it's limit. To this point nothing has gone up in smoke. Maybe a simple fan would help w/ the heat and don't mess w/ Murphy's Law? I thought I'd run it by you guys-

[catnine]

 I can tell you this much . I built a parallel SE 6V6 of the same order as the fender 5F2-A and used PT of the same rating as the one you want to use and I could remove the two 6V6's and plug in one 6L6 and had a huge allen amps OT . In the end I really heard nothing as to louder just a bit cleaner with the 6L6 . To add the larger PT ran hotter than the smaller PT as a 6V6SE princeton and I needed a fan , this is to say i could not keep my hand on the PT .

[tubeswell]

I did a parallel SE for 2 x 6V6 etc or 1 6L6/KT66 etc and used a weber 025130EU PT I had lying around in my stash. These PTs have a 350-0-350 HT secondary with 280-0-280 taps. I quite like the sounds I get using the 280-0-280 taps with a single 6L6 and a 5U4G rectifier. (The plate voltage was around 325IIRC) The voltage notes for this amp using different HT secondaries and rectifiers and output tube combos is attached FWIW) - along with a schematic (so you can piece it all together and figure it out).

[plexi50]

I would make the change to the 330-0-330 PT. I love low wattage amplifiers but there is something magical at the 430-460 B+ range that makes them EL34/6L6GC,KT77's sing. With a 280VDC plate there is a lot of headroom you just cant qet.  My Peavey Windsor Studio is running a single ended OT with a 6L6GC or EL34 @ 455 VDC plate. It sounds great but i had to make a lot of preamp changes to this amp. Some times i think of the old Gibson Kalamazzo and how full and sweet it sounds. I think theres a lot to be said about preamp cathode resistor values,coupling caps and tubes besides using a 12AX7. Some of those old single ended low wattage amps just sound fenominal. EF86,6SL7,6EU6

[tubenit]

Common sense says to leave it alone and don't screw w/ things that are working and sounding great.

Most of the mods I've made have worked out OK.      Some haven't.

Sounding great is a pretty cool description, IMO.

With respect, Tubenit

[jojokeo]

Thanks for your input everyone. I was doing a bit of tweaking changing & testing speakers on a past se ef86>12a_7>el84 amp from a couple of years ago and was surprised at how loud it is. Compared to this el34/6l6 amp they are actually pretty close in volume. I would think this amp should be louder and closer to a couple of pp 6v6 amps I have or even a pp 6bm8?

I would like to get as much volume & headroom out of this amp as possible and use it in smaller clubs if it's possible. So my big question is will I get more volume and headroom w/ higher plate voltage closer to 400v's over the existing 250v/100mA place where I'm at now???

I could re-take preamp voltage readings & adjust for this after a PT replacement to get things back to where they are there. And it has a VVR so I'd always be able to adjust the power tube down and get voltages & tone for where it currently is at. So if I was to shoot for 400v say, then for 25w Pdiss my current rating requirement would go down to 62.5mA minimum. My spare PT is too large physically and at 125mA is double what's needed. I may have to find something for a better fit if I go this route.

Plexi or anyone - have you ran a el34/kt77 (or the like) at the lower & higher voltages that can verify a pretty good volume difference or is it more simply a matter of mainly headroom?

[HotBluePlates]

What is your present B+ (with the 260v tranformer) and what is you OT primary impedance?

[jojokeo]

What is your present B+ (with the 260v tranformer) and what is you OT primary impedance?

Last I measured using 150ohm bias resistor w/ an el34 to get to 25w Pdiss it dropped to 250v. so I assume my Ia is close to 100mA. The OT is a 5K:8r & 16r and I'm plugging into the 16r tap w/ an 8ohm speaker to drop the pri Z to 2.5K

Before I closed it up for the last time I found a 200ohm bias resistor & installed it thinking the Pdiss would lower a bit to keep the tranny from being all the way maxed out. There really wasn't any noticeable change to tone or power w/ that change.

[PRR]

> EL34/6L6-5881/KT77 ... When I switch to the various tubes there really isn't much if any perceiveable change in output volume.

Why would there be? Any of these tubes, with reasonable load, can swing ~~80% of supply. Maybe 77% or 85%, give or take. Same supply, same load, all will make around the same output.

> 6l6 or 5881 ... should only yield about 61/2 watts.

Marketing. The original 6L6 suggested conditions were never changed as improved 6L6 flavors came into production. (RCA and Tung-Sol wanted to sell other types at higher prices; also there was near-zero commercial interest in SE over a few watts, the OT costs too much.)

It is well known that one modern 6L6 well fed and loaded will make around 10 Watts.

> a parallel SE 6V6 ... plug in one 6L6 .... really heard nothing as to louder

One 6L6GC will do the work of two 6V6. Two 6V6 allows 24W-28W Pdiss, one 6L6GC (any new-made "6L6 family) will handle that.

> 150ohm bias resistor ... 200ohm bias resistor ... wasn't any noticeable change to tone or power w/ that change.

The cathode resistor has to change a lot to get a little effect on current. 150 to 200? You probably dropped from 100mA to 90mA. Power output changes even less. Ideally the load should change 10%, but real speakers are ALL over the place, more than 20% "off" at any frequency.

If you have too much money, you could have a Hammond 125ESE shipped to you. Working as 2.5K it will handle 100mA fine. (At 2.5K it will do 38Hz with the rated 80mA; 100mA will start to saturate but soft and it will still pass 50Hz, well above guitar zone.) But it's pretty expense (iron plus shipping) and I doubt it will be a real difference.

[jojokeo]

PRR you have a way of keep things in perspective and at the same time dummying down the technical which is always refreshing. You're confirming my suspicions on some things and validating others.
Now I made one last change today concerning the output tranny's load.
With my new HT measurements & Pdiss findings I used this formula for calc'ing out the load to use.
Pri Z = V^2/W: The el34=3.6K, kt77=3.5k, and 6l6=3.4k
EL34 example: 277v HT and Pdiss = 21.43 w which equals 3.6K (<--is this correct for actual pri Z load wanted?)
I "was" using the 5k tap on the OT and running an 8ohm speaker into the 16ohm tap to get the 2.5K impedance. But w/ my new findings I changed to the 8k pri tap doing the same w/ the speaker & 16ohm tap to get a 4k pri Z load since it's closer to the new measurements above. Did I do the correct thing here because you referenced the 2.5k load being used?

[PRR]

How does it sound now?

But short answer: SE EL34/6L6 will barely work with a live drummer unless he is unusually restrained. To have a hope, you go all the way to the 30W Pdiss and use a VERY efficient loudspeaker. Light-cone Twelve (or two!) with a good magnet. (Not the soggy Twelves sold for hi-power shrill transistor amps.)

[jojokeo]

I didn't get a chance yet to run it all out. First initial impression at very low vol was a very slight possible lack of highs/clarity but it could just be the high resistance of the vol pot turned so low (no treb bleed cap)? I'm not sure if that would even be a characteristic associated w/ this or not? I'm running a UK 20watt Celestion Heritage Greenback that really sounds sweet w/ this.

"To have a hope, you go all the way to the 30W Pdiss" - el34 is 25w Pdiss rated in spec sheet so I thought I was good to go? Hmmm, I can go down to the 150r cathode resistor but then will my tranny start to turn into a space heater? When I'm playing all the way up for a half hour or more, I can barely keep my hand on the thing the way it is now.
This was what got me into thinking of making that tranny switch to raise voltage to lessen current & heat. I was hoping of using it in small clubs and practices on occasion but the last thing I need is to be part of the light show. I guess my big lesson learned on this one is to give the PT a bit of extra horse power because I designed this one right down to the gnat's ass. The output on this may be about 8 watts or so as even though I haven't directly compared the two, it seems fairly close to two 6bm8's of another amp in the stable.

[jojokeo]

A quick update on the pri Z: the 2.5k is slightly but noticleably punchier, cleaner, and seemingly a bit louder than the higher 4k load even though the 4k was closer to the measured values to use.

What is it about the loading effect used being a bit lower than calcualted or suggested that gives better performance & tone? I'm finding this trend in a few amps now and that there's a correlation here. Just like power tubes being pushed running a higher bias or Pdiss, the increased loading seems to yield yet another positive effect.

[PRR]

> What is it about the loading effect

It's complicated. Also depends on the tube and the speaker.

Use V/I to get close, then use your ears.