Working with a Hammond 270HX 550VCT.. Need to get 450V B+

[J M Fahey]

The 230mA rating is dependent upon drawing current from just one side of the centre tapped winding at a time. If you use a bridge rectifier across the full winding the current rating is effectively halved to 115mA. You MUST then leave the centre tap NOT connected. Also the 115mA current rating you end up with wont' cut it for a pair of EL34 in push pull so you need to change your aims or go for a different transformer.

A pair of EL34 cathode biased at around 340-350V B+ is good for 30 to 35 Watts and in my opinion at least sounds MUCH better than running EL34 in fixed bais at B+ of 450V plus trying to screw 55 watts put of them. I'll take the lovely 30W in lieu of the agressive 55 watts anyday. OT with Raa of 3K4 top suit.

Cheers,
Ian

This is the best advice you found so far.

In the real World, your 388V powered amp will sound and feel the same as a +450V , and will push almost the same as the other .

We are talking a 1.16X voltage difference, which means 1.3dB volume difference= nothing, almost inaudible.

Do not turn it into a plain watts difference, because then it will look larger ..... on paper ...... ; keep it in dB to better grasp the real meaning.

In fact, use the transformer you have, no need to vary impedances or nothing.

And it will be *slightly* easier to get power tube crunch onstage.

*If* some day you get the "proper" transformer, fine, switch it .... althougn you won't find that much audible difference either.

[Mick Bailey]

I'm not seeing the Trainwreck pics - is it just me?

[Gingertube]

I'm not seeing the Trainwreck pics - is it just me?

Maybe my file sizes were too big - lets try again

Cheers,
Ian

Edit - no maybe about it, it was file size - The critical thing for the above discussion is the EL34 biasing, individual 270 Ohm cathode resistor or each tube with 200uF of bypass on each - VERY hotcat.
The parallel triode input stage maybe of interest to some.

[J M Fahey]

Thanks.
It must be very touch sensitive.
As of "hot", at 31W dissipation per tube, I do not have any doubt about that

[Zozobra]

Maybe my file sizes were too big - lets try again

Cheers,
Ian

Edit - no maybe about it, it was file size - The critical thing for the above discussion is the EL34 biasing, individual 270 Ohm cathode resistor or each tube with 200uF of bypass on each - VERY hotcat.
The parallel triode input stage maybe of interest to some.

Thanks. The old one wouldn't load. I'll have a good look over this

[ReadyTeddy]

Maybe my file sizes were too big - lets try again

Cheers,
Ian

Edit - no maybe about it, it was file size - The critical thing for the above discussion is the EL34 biasing, individual 270 Ohm cathode resistor or each tube with 200uF of bypass on each - VERY hotcat.
The parallel triode input stage maybe of interest to some.

2x the tubes = 2x the transconductance. Is that the reason you used the parallel triodes? Or, 2x the tubes = .707x the noise--maybe that's the reason?

[Gingertube]

I'm thinking that the approx 24V I wrote on the schematic was measured while driving continuous sine wave at 32Watts RMS into a 4Ohm resistive load (just below clippling). At idle it would be more like 20V (75mA). That is still about 27 Watts dissipation per tube. I'm running NOS Philips 6CA7 in it and they seem quite happy. At a B+ of 345 to 350 from a Hammond 270HX that will come down to 26 Watts.

If worried, take the cathode resistors up to 330 Ohms each (200uF bypasses)
OR if you want to try one of Ian's "fancy ar...ed" odd harmonic and intermodulation distortion reduction schemes, use fully bypassed 220 Ohms on each cathode with a common 39 Ohms (unbypassed) below them back to 0V. That introduces some common mode feedback in the output tubes.

http://tdsl.duncanamps.com/dcigna/tu...ex/6ca7-4g.gif

Says 2 x 95 mA at full power, 2 x 75 mA at idle for B+ of 375 and common cathode resistor of 130 Ohms (I have B+ of 380V and 270 Ohms each cathode).

The other thing I have not updated the schematics to show is 22K grid stops on both sides of the Phase Inverter, the damn thing was busrting into oscillation when the volume was turned above about 6 or 7. These grid stops fixed that.

The parallel triode input was an attempt to give some harmonic richness by operating the two triodes at dissimilar operating points ( I think it does this well). It would help noise marginally (0.7 as you say), besides there was this spare triode section doing nothing, so I put it to work. It also drives the tonestack a little better (lower Zout).

Cheers,
Ian

[thehoj]

I have to say this thread has been informative and interesting, I appreciate all of the information.

That being said, I didn't end up going this route, and instead had a different amp more suited to my needs kind of fall into my lap, so I won't be actually building anything around the 270HX.