SE fun build idea - comments please

[tubeswell]

I have posted this over at Ampage and Hoffman as well, but absolutely know that you good people will have something helpful to say about it.

I have a PT with 240-0-240 100mA and 6.3V 2A and a fairly chunky 25Wish SE gapped OT 8k Pr with 4, 8 and 16R secondaries, plus the rest of the bits I need to get this rolling.

I've fancied a parallel SE amp for some time and also like the idea of an EF86 with Merlin's triode/pentode morph control. I also dig trem and have a spare triode and a pile of IRF820s so...

Here is the 1st cut at the schematic. Any suggestions/comments/criticism feedback is welcomed please. (The voltages are just guesses)

I am particularly interested in whether anyone here has any reckoning about adding NFB (none at present on the schematic), and also the relative sizes of the trem depth pot and the output stage grid load resistor (highlighted in Blue) - I was wondering whether a 100k pot and a 180k grid resistor would be more appropriate, or maybe a 100k pot and a 220k resistor?

TIA

[ToneMerc]

I think you are being very liberal with the expected voltages using a 240V/100mA transformer. Your PT is undersized for a parallel SE application. While 100mA is fine for a single 6V6, it's barely marginal at best for two parallel 6V6s and definetly not enough for two 6L6's. Let's look at the PT from a duty rating perspective at the common duty rating factor of 75%.

(240V X .10A) X .75 = 18W

Two parallel 6V6s in Class A will be around 12-14 watts each.

Also, 240V is only going to supply about 335V or so unloaded on the plates. Once you put a load on it, I believe your plate voltage will only be in the 285-290V range.

The PT that I'm using with two parallel 6V6s, a 200mA choke and a GZ34 rectifier, is 360-0-360 @ 150mA.

TM

[tubeswell]

Thanks TM, I have another PT closer to the spec you mentioned which should do the job if things get rough. The PT is probably good for 340-350 B+ because it is custom-made by a local guy I know, and his PTs are bigger than usual and seem to handle a bit more power than commercial stuff. Actually I have a couple of old chokes that somewhat might be good for a CLC supply filter for it as well, but I am only going by the size. One has markings "4415-026" and "01855" on it, and both measure about 1 Ohm DC resistance - both of them are wound with really thick gauge wire (about 18-20 ga?). The little 22707 type fender choke in the pic is just there for a size comparison (and I know that would work for a CLC supply choke with a 1 x 6V6 in SE, because I did a standalone reverb with one)

[tubeswell]

Chassis finished yesterday. Pics here: http://music-electronics-forum.com/t23443/

[M Fowler]

Very nice, now going in a tweed cabinet?

[statorvane]

Super job on that amplifier, especially the chassis. Having bent one, I thought I'd never do it again, but have been contemplating getting a brake. What kind of brake is a bar-brake?

[tubeswell]

Very nice, now going in a tweed cabinet?

Yup. I have one 90% complete. I'll post some pics when I've finished it (maybe later today).

[tubeswell]

Super job on that amplifier, especially the chassis. Having bent one, I thought I'd never do it again, but have been contemplating getting a brake. What kind of brake is a bar-brake?

Its almost the same as a box and pan brake to look at but it doesn't have the separate 'teeth' (on the brake part that clamps down onto the anvil) - instead it as a solid bar. I think it works better for the kind of chassis I bend, because it has nice straight edges. Pics of it here

http://music-electronics-forum.com/t21746/

[M Fowler]

To do the sides isn't it necessary to make or find a finger plate so get into that small area?

The brake does the sides but to get into the ends the fingers allow that.

[tubeswell]

I have developed a method for doing the endcaps in my vice so I can get away with it.

[tubeswell]

Oops

[tubeswell]

Well after a mild kerfuffle this morning, (insanity break caused by troubleshooting mis-wiring and extending the height of the board off the chassis to avoid unwanted shorts), I figured out that the way I have the trem set up must be interferring with the 6V6 bias something wicked, so back to the drawing board for that part - looking for ideas please (maybe pre-amp driver tube trem wiggle??).

Anyhow I'm getting past myself.

I disconnected the trem and sorted out the NFB polarity and decided to swap over to the 'low power' HT taps on the PT. Still at this stage with one 6V6 plugged in running off a 5Y3GT. I also 'upped the 22k resistor to 27k, but I might put that 22k back again.

Anyway my first impression are that the 'Mojo' control is really effective at going from pentode to triode mode, and it has a wider range of tones and sounds than I wasn't expecting from a tweed princetony style amp - what with the 'Mojo' control and the volume and tone, there's quite a lot of scope with this little baby. And its loud - I couldn't turn it up past 4 with the little 15W alnico something speaker I found in the wardrobe. Scoops first prize. I will wire up the Greenback G10 later and try out the 2nd 6V6, although I have a hunch I'l be going to a 5U4G or GZ34 or going to those high taps.

I must get some soundbytes going.

PT
HT 558VAC (279-0-279)
Heater – 6.79VAC
Rectifier – 5.27VAC


Loaded
5Y3GT, 6V6S (8K Pr Z), 12AX7, EF86

HT voltages
B+ = 326 VDC at reservoir cap
10k node = 275 VDC
27k node = 209 VDC

6V6S
Plate = 321 VDC
Screen = 272 VDC 560R g2 resistor dropping 3V = 0.005A
Cathode = 16.4 VDC (across 472R measured) = .0347A

.0347A Ia+Ig2 x 304.6V a-k = 10.6W (minus 5.3mA Ig2 = 9.5W plate dissipation)

12AX7

Driver Stage (209 HT)
Plate = 127 VDC
Cathode = 1.23 VDC

LFO Stage (343 HT)
Plate = 155 VDC
Cathode = 1.75 VDC

EF86
(209 HT)
Plate = 92 VDC
Screen = 69 VDC
Cathode = 0.99 VDC


Voltages on the 'high-power" HT tap:


PT
HT 719VAC (359.5-0-359.5)
Heater – 6.79VAC
Rectifier – 5.27VAC

Loaded
5Y3GT, 6V6S (8K Pr Z), 12AX7, EF86

HT voltages
B+ = 415 VDC at reservoir cap (470 peak at startup)
10k node = 343 VDC (436 peak at startup)
22k node = 280 VDC (369 peak at startup)

6V6S
Plate = 406 VDC
Screen = 340 VDC 560R g2 resistor dropping 3V = 0.005A
Cathode = 19.8 VDC (across 472R measured) = .0419A

.0419A Ia+Ig2 x 386.2V a-k = 16W (minus 5.3mA Ig2 = 14W plate dissipation)

12AX7
Driver Stage (280 HT)
Plate = 182 VDC
Cathode = 1.31 VDC

LFO Stage (343 HT)
Plate = 208 VDC
Cathode = 1.94VDC

EF86
Plate = 120 VDC
Screen = 91 VDC
Cathode = 1.3 VDC

[tubeswell]

Okay, what I am thinking now is to aim to drive the output tube grid into cut-off by making the trem swing into really -ve voltage. (The PT has a 45VAC bias tap). I don't know if this will work, but please bare with me, and please please do chime in if you know how it could be done a better way etc.

Anyway the theory is that at present the grid sits at 0V (being a cathode-biased output tube'n'all), and I think that if I design the trem to swing only in the negative-voltage range, with a concomittent -ve bias adjustment on a dual-ganged pot (that is the same pot as the trem depth control), that I could sort of match the amount of -ve voltage required at the trem voltage swing mid-point to be set relative to the trem depth, so that the trem's most-positive swing was always at (or near enough to) 0V, and the output tube was driven into really cold bias, so that the trem doesn't adversely affect the normal biasing of the output stage (as it otherwise appears to be doing after I bench-tested a prototype of the previously dreamed-up incarnation). Please refer to the concept schematic* attached. TIA

* in the concept schematic the light-grey shaded line is meant to represent the dual-ganged pot, which in theory when turned CW both increases the trem depth and increases the -ve voltage of the trem swing mid-point)