Rectifier help

[PaulD]

With just one EL84 the transformer will be adequate, the typical current requirement for an EL84 in SE cathode biased amp is around 48mA and each of your preamp valves will need around 6mA. However the transformer must be wired with a full wave rectifier and grounded centre tap, in this configuration you should expect a B+ of around 250v (RMS voltage 190 x 1.4 = 266v but in practice this will be slightly less under load and after filtering, RMS x 1.3 is usually a good approximation). When you wire it to a bridge rectifier as you have done you double the unloaded voltage but halve the available current (there is no such thing as a free lunch!) so you would only have 35mA available which is not nearly enough, this explains why your voltage is not going massively higher with the bridge rectifier, if there were enough current available it would have increased to around 500v!

[martin manning]

I'd say 35mA at 350V Va-k, with 5k plate load. If you have the Hammond 125ESE OT, it is good for 80 mA, and you can get a 2k5 load for two EL84 if you switch to a bigger PT.

edit: corrected load impedances

[sixstringer]

OK! with the rectifier circuit change per Martin's modded layout, the voltages are now within a couple of volts of those on the original schematic. Thanks for your help!
Do you have any advice on how to get this circuit to be more "chimey" and no so mid-range heavy? I am hoping to get more that EL-84 glassy tone (I'm note sure how to describe it)
that I get from other amps, especially Vox and Matchless style circuits.
Again, thank you so much for your help and advice.

[martin manning]

First, as far as I know the schematic I posted is the original. What voltages do you have on the EL84 now? Can you get a reading from all the pins to ground? And, what is your cathode resistor value?

[thetragichero]

more chimey... could that be as simple as lowering that first cathode bypass capacitor from 4.7uf to 100nf? i like using that value for vox-type bright channel

[sixstringer]

Power Tube (EL-84 ) voltages:
Plate (pin 7) 257vdc
Cathode (pin 3) 6.5vdc
Screen Grid (pin 9) 249vdc
Control Gri (pin 2) .137vdc

Voltage drop across the Cathode resistor: 6.5vdc
Cathode resistor value is 120 R

267 vdc (B+) from the rectifier

[PaulD]

Assuming those voltages are with one EL84 then they indicate a plate current of 51mA and power dissipation of 13 watts. A little on the high side but not out of the ordinary, many amps routinely run EL84's above their recommended ratings but if you wanted to bias it a bit cooler using a 150 ohm cathode resistor would probably drop that to around 12 watts dissipation which is the recommended maximum for an EL84.

As for the Vox chimey sound that is usually associated with a push/pull cathode biased EL84 amp and also has a lot to do with the Celestion blue alnico speakers, unfortunately a single ended amp is never going to sound like a Vox AC30! Tweaking some preamp components for a brighter tone may get you closer to what you want but SE EL84 amps always tend to sound a bit "fizzy" when driven hard and there is not much you can do to prevent that.

[martin manning]

I get 6.5/120 = 54.2 mA at the cathode, and I'd reduce that by 10%* to 48.8 at the plate, making the dissipation (257 - 6.5)*0.0488 = 12.2W, which is probably fine. The OT load should be 5k (corrected my numbers above) for center bias.

*You can calculate screen current using the screen resistor if you want a better number.

[PaulD]

Have to admit to cheating a bit and using an online bias calculator which I guess was using 5% for screen current. 5mA or so is probably closer to the mark for a SE EL84 circuit like this so yes, probably OK.

[sixstringer]

With the 150r bias resistor, the plate dissipation is down to 9w, which seems very safe for a JJ 6v6 (14w max) in a class A application. Also, I changed the bypass cap on V1 to improve the highs and I like it better. The amp is working fine.
Thanks for all your help and advice.