Transformer choice for 6V6 Express

[Firestorm]

Kevin O'Connor says the same thing (and makes merciless fun of Weber in the process). KO'C basically holds that impedance matching is a "tone control" and you can, within reason, play with it to your heart's content. The exception that everyone notes is old Marshall transformers, which were, evidently, not quite up to the task of running at "correct" impedances.

I think The Marshall note was about output impedance mismatch.

Yep. Questionable insulation that gets punched through.

[Tone Lover]

8K Raa is the book spec and I have built a few guitar amps with various output trannies (with an Raa of 8K) ranging from Hammond 1608 to cheap chinese jobs (see link below), all sounded fine.

The 6K6 (see rp's post above) would also be OK, I haven't built an amp with a 6K6 Raa output tranny BUT I have built one with a quad of 6V6. In that case I used a JTM45 Output Tranny with an Raa of 3K4, that was gorgeous (3K4 for a quad is same as 6K8 for a pair).

Latest 6V6 PP Amp I used one of these:
http://www.mableaudio.com/en/productvie ... ormer.html
It is a 8K Raa HiFi Tranny with a rating of 12 Watts RMS intended for a pair of EL84 in Ultralinear Mode. Since we are not trying to get much bass out of it for the guitar amp you can use it at more than the rated 12 watts. It worked fine at 15 Watts from a pair of 6V6 in Pentode mode. Just tape and stow the unused Ultralinear taps. One of the guys who has tried out this amp was so impressed he has asked me to build him an identical amp with the identical output tranny (luckily I have 2 of these trannies, I pulled them from a EL84 HiFi Amp and replaced them with Hammond 1608, the Hammond was superior for HiFi, the Chinese jobbies are just fine for guitar since we don't need extended tops or bottoms).

Cheers,
Ian

Gingertube I think that one you linked to says its a 10k tranny which could explain why it sounds better,
I now use a 4k2 in my EL34 marshalls because it just sounds better to my ear than the standard 3k4 or 3k3 thats said to be stock in those. To my ear that gives me more compression and bite with the distortion. I built two plexi,s same exact parts exept for OT everyone that played it chose the 4k2 ,I guess I should add both were magnetic components transformers so it wasnt a mismatch of transformers.
Could be the higher impedance sounds better for the EL84 also.
Bill

[Tone Lover]

Forgot should give credit were due thanks Realtarded for the 4k2 EL34 tip.

[M Fowler]

Yes I sometimes use the Dynaclone A-470-S 4k3 OT but I'm not sure using 4K sounds better, but we all enjoy different tones and our ears are not all the same.

Mark

[gingertube]

I had not noticed that the chinese jobby I linked was 10K. That could indeed be why it sounds good, although its medium quality features (some interleaves of winding sections) is probably a better reason.

In general (so the theory says) increased load impedance will give slightly less power and slightly less distortion. You can get the power back by using a higher B+ (which is to say that a higher primary impedance also is perhaps a good idea when running higher B+ which it seems 95+% of amp designers do (higher B+ that is not higher Raa), to screw the last watt out of it no doubt).

I had 2 of those Raa = 3K4 JTM45 Output Trannies that I talked about above. One as stated was driven with a quad of 6V6.

The other I used for a Trainwreck, so it is appropriate to mention it here. I used cathode biased push pull 6CA7 at deliberately chosen lower B+ of 380V. That gave a max of 32 Watts but they were glorious watts. I can well believe that 4K2 or 4K3 would better suit EL34/6CA7 at say a B+ of 450V.

In all of the amps I build these days I am using very conservative B+ voltages and current swings. That means using book value Raa with book value B+. If I have higher B+ then I use a higher Raa tranny. The amps just seem to be more relaxed and effortless when the tubes are not being pushed to their limits. Of course it may also be that I'm now pushing hard up against the south side of 60 and am accumulating "old fart" status and opinions.

Cheers,
Ian

[JazzGuitarGimp]

The only modern-day builder that I know of who is conservative with output power is Fred Tacone (Divided By 13). Here are some examples from his website:

EDT 13/29, 29 watts from a pair of KT66's
RSA 23 and BTR 23, 23 watts from a pair of KT88's
LDW 39, 39 watts from a pair of EL34's
AMW 39, 39 watts from a pair of EL34's

I have always suspected his recipe for sweeter tone stems from conservative output power.

[barfoden]

Interesting thread..

Primary impedance values seem to differ a lot.
One guy reported that he preferred a Push pull 6v6 amp with 415V plate and 410V screen with a load of 4K. He first tried to double the value of the speaker impedance by moving the 8 ohm speaker to the 4 ohm out so the pair of 6v6 should see a reflected impedance of 8k which is more textbook numbers.
But the 4K load sounded better he said, more bandwidth and less compressed. The princeton reverb used a 8K load and the deluxe reverb a 6.6K load both with around 415V plate and screen voltage.

Another guy was using a 2.6K load on a pair if 6v6's at 420V plate as he felt the 6.6k load was ice-picking. But i suspect that his problem lies elsewhere in his circuit.

The Marshalls EL34 plexi with diode rectification and the 3.6K OT should be used with the 625V/150mA PT and not the 690V/150mA PT. The B+ under full load (150mA) is 456V and 412V respectively (classictone homepage values). You have two problems with the high voltages, If biased relatively hot, you will risk to get beyond the plate dissipation range somewhere on the volume dial, and the EL34 screens do not like high voltages. This is why you need at least a 1-2K screen resistor to prevent the screen from drawing to much current.
You encounter the same problem with vintage blackface Fenders running beyond schematic voltages due to modern wall AC.

One repair guy had to install 1K screen resistors to his clients vintage twin reverbs to prevent the screen on the 6L6GC from burning up. The schematic value shows a plate voltage of 460V. Say the wall AC volts are 125V instead of 115V the plate voltage will sit around 490-500V plate. The plate might take the extra heat but the screen voltage sits way to high.. Should be well below 450V. Fender was allready pushing the 6l6GC to their limits to get as much output as possible. Higher primary voltages will put them over the limits. Also your filament voltage should be checked.

[Reeltarded]

The other I used for a Trainwreck, so it is appropriate to mention it here. I used cathode biased push pull 6CA7 at deliberately chosen lower B+ of 380V. That gave a max of 32 Watts but they were glorious watts. I can well believe that 4K2 or 4K3 would better suit EL34/6CA7 at say a B+ of 450V.

Cheers,
Ian

In conversation low B+ is all the rage. I have used a variac for so long as a volume control that I am pretty sure 380v and 4k2-4k4 OT is a magic match for a well tempered amp. I am least worried about output volume. Anything under 30w is somewhat unworkable with a strong drummer and anything over 50w I am variacing to about 75v with the same.

I think i like lower voltages but biased up to the line anyhow.

[M Fowler]

Build as clean of an amp as you can for max clean headroom and use pedals galore to get all your color splash. This allows the purest signal and you don't have to use attenuators.

Or short version use a bass amp with pedal board in front perfect tone.

[Reeltarded]

hahaha

[Mark]

Perhaps I haven't quite understood all that has been said here. There seems to be one group liking half the primary impedance (4K instead of 8K).
At one stage doubling the output impedance was discussed (in this case a quad of 6V6's would run at 8K), and finally running the tubes somewhere between 10 to 50% greater than the accepted impedance.

I seem to recall K.O'C mentioned in one of his books about getting 100W girth from a 50W amp by using four EL-34's. I wonder if this could be applied to an Express and would it sound good?