PT size and reality

[WRC34]

drew, this is interesting. I read Merlin's article a few times and it looks like if I were to employ this method I would:

a) have to use diodes that "have an average current rating that exceeds the heater current" - it seems like this would require a very beefy diode. The 1N5408s I have are rated for 3A whereas the commonly seen 1N4007 is only rated for 1A. But in an AC30 I would be past 3A, so what type of diode would suit this application? Would something like this work? http://www.jameco.com/1/1/35699-50sq100 ... 204ar.html

b) the diodes would be used on one leg of the 6.3V winding BEFORE the virtual center tap (since this PT has no center tap on it's 6.3V winding, a virtual center tap is necessary)

[pdf64]

Yes to point 'a'.
But I think that schottky diodes have a very low forward voltage drop, so aren't suitable for this application, as the whole purpose of the diodes is to drop a little voltage.
I think that adding resistance would work out easier in this case; especially the aluminum cased chassis mount type.

[drew]

Merlin's page previously showed 60S1 diodes as an example. Don't know why he removed it. When I did this, I went to an electronics store and asked for those; they didn't have them but had some even beefier ones. Overkill for a Beast Champ, but they work.

The bridge rectifier setup he shows would be an easy way to go. If you still have a Radio Shack somewhere nearby, the stores are still stocking an 8 amp/400 volt bridge rectifier, p/n 2761181.

[lumox0013]

sometimes they are just small seemed to work in this app.

http://www.tc.umn.edu/~jward/mel2.html

[JMFahey]

Thinking laterally:

If you are so tight with your budget, why not just build a *two* EL84/6V6 amp?

Ample power for most uses, you use what you have at a sedate pace, (amp will last forever) and anyway most people find their current amps too loud and start endless threads on how to tame them ... you'd start with the right foot

It would be real sad to spend double your current budget and then worry about taming it down.

Just sayin'

[WRC34]

JMFahey, though my finances aren't without limit it's really my time that's on a tight budget.

I have been stockpiling PTs and OTs for a couple of years and I have enough iron to make three healthy two EL84 or 6V6 amps. This amp will be for personal use and, as stated in the first post I want to tackle the elusive EL84 quad amp that gave me so much trouble before. I have an old Allied branded 35 watt OT made by Northlake w/a 6.6K primary and 4/8/16 secondaries. It has the same physical dimensions as the TW
OT offered by Classictone. The temptation to use this in a higher powered, 30 or so watt amp is just too great! Volume won't be an issue, and the less tame the better (to a point of course).

Also - would using fewer tubes on the heater supply will increase the need to drop heater voltage? If I build this amp with one three 12AX7s and a quad of EL84 the heater current draw will be around 4A, a little less than half of the winding's rating. So I'll definitely have to pull the voltage down with resistors or diodes, but less so than if I used fewer output tubes?


drew & pdf64 - I'll keep my eyes peeled in my travels for a Radio Shack or electronics store. Maybe I'll get lucky. If not, I can resort to eBay!

[WRC34]

Tons of typos = iPhone vs. laptop!

[WRC34]

Reviving my own old thread w/build update here.

I ended up using this PT for a cathode biased EL34 amp instead, just finished the build/chassis. Photos are attached.

I have no schematic but here are some stats:
-tubes are 6SJ7 input, 6SL7 ltp phase inverter, EL34 outputs, 5AR4 rectifier
-Filtering is 47uf > 30uf > 22uf > 16uf
-EL34 plate voltage is 340VDC, screens are 10 volts lower, 330VDC. Uses common 470R 5W screen resistor
-output transformer is 6.6K > 4/8/16 manufactured by Northlake probably in the 1960s
-Thordarson T57C54 choke, 10H

Despite the seemingly low plate voltage the thing breathes fire. Loads of sustain and overdrive. Nice crisp high end. It does have that, uh I don't know exactly how to describe it but I guess you could say "roundness" or "smoothness" that cathode biased amps are known for, but just enough. Maybe it's just tube sag I'm trying to describe?

Overall very happy with how this turned out!

[WRC34]

oh! also -

-heater voltage turned out to be around 6.6VAC (measured as 3.31VAC on either side of any given tube's filament)

-separate 270R 10W cathode resistors are used, each bypassed by it's own 220uf 100V cap

-coupling caps are .022 from 6SJ7 plate > .01 from vol pot > .025 into EL34 grids (those big Russian grey ones)

[rp]

Not sure what the best Rk is for your set up, best is to do the dissipation math, but I'd think you want 35-50mA on each tube, google the Weber bias calculator. Here's a good post on cathode biased EL34s.

http://el34world.com/Forum/index.php?topic=15615.0

[WRC34]

rp, thanks for your response!

I have seen, and even used as a guide in this build, info the Hoffman board guys have posted about cathode biased EL34 amps. The thread you shared is cool. There is also another one here -
>http://el34world.com/Forum/index.php?to ... #msg139977
- where "sluckey" lays out some pertinent info regarding the importance of observing screen current and it's dramatic difference from the few negligible volts usually seen in 6L6 types.

Using his suggested method in Reply #16 I measured 11.85VDC across a shared 470R 5W resistor which calculated to 25mA screen current. Subtracted from the 78mA cathode current measurement I obtained by using Jojokeo's method earlier in the thread (taught to him by geezer the gate keeper of old knowledge) = 53mA. It seems I'm either right past the limits you proposed (35-50mA) or I'm just under the max plate dissipation spec at 24.994 watts per tube dissipation, which is 78ma multiplied by plate -to-cathode voltage of 317.

http://el34world.com/Forum/index.php?t ... #msg139977

[WRC34]

and happy new year to all!