Simple little 6V6 amp with very low output (schematic and measurements included)

[mac dillard]

Some strange numbers in that amp...Jumper R1 or R2 out and see what you get. They are sometimes as low as 10K, Also R3 of 100K is strange. I would expect 1M there. Jumpering R1 or R2 will not hurt anything except you may pick up a radio station.

[DrGonz78]

Yup R3 needs to bleed less guitar signal to ground. I am curious if it helps get a louder signal through the amp at least. Good eye!

[mac dillard]

The R1 or R2/R3 divider would give you 0.25 volt input..

[g1]

The Hammond "Design Guide for Rectifier Use" specifies average rectified DCV for full wave rectifier as 0.45 x VAC, where VAC would mean 480V here.

Hammond is known to have some glaring errors on their website. If that formula were true, we would have less than 300V B+ when using 300-0-300 HT windings. What kind of winding would we need to get 450V B+ ?
I think that formula should be peak, not RMS.
So for this amp, maybe expected 300VDC less 5Y3 Vdrop. I guess 240VDC is still ballpark if 5Y3 is giving 60V drop.

[g1]

Where/when did you have a 25Hz grid?

Niagara region (southern Ontario where Hammond transformers is located and upper New York state). Was still in use into the 1950s.
Some interesting history:
https://www.lifebynumbers.ca/history...ty-in-ontario/
https://en.wikipedia.org/wiki/Utilit...#25_Hz_origins

[Helmholtz]

Hammond is known to have some glaring errors on their website.

I'm aware, so I crosschecked with standard literature.
That factor of 0.9 per side gives the lower average DC limit for vanishing filter capacitance or high load current, where voltage drops to zero between recharging.
So would mean excessive ripple.

But for small PTs with high effective DCR, the DCV can approach the (no-load) RMS AC per side.
According to Merlin, DCV can be equal to ACV per side with smaller PTs and tube rectifier.
In his hifi book he states:
" In other words, a 300-0-300V transformer with a valve rectifier is likely to produce close to 300VDC at full load"

A class A amp idles at full power.


That's why I asked for actual (loaded) HT voltage.

[Helmholtz]

Niagara region (southern Ontario where Hammond transformers is located and upper New York state). Was still in use into the 1950s.
Some interesting history:
https://www.lifebynumbers.ca/history...ty-in-ontario/
https://en.wikipedia.org/wiki/Utilit...#25_Hz_origins

Very interesting indeed.
Just reminded me, I regularly (twice a year over 10 years total) met the Canadian utilities representative in power quality IEC meetings.
He also organized the Montreal IEC meeting (sponsored by Hydro-Quebec) I attended.

[Helmholtz]

@g1,
that's the guy I meant (Roger Bergeron), i.e. the author of this paper: http://www.electrigenies.com/PQTutorial.pdf
(Sorry for derailing)

[Helmholtz]

I used PSUD2 to simulate the power supply/rectifier circuit.
With a load current of 40mA, effective PT resistance of 400R and a 5Y3 rectifier I get 239VDC from the 240-0-240 PT secondary.

[walkman]

Nothing wrong with an idle current of 33mA here, see datasheet.
Too high an idle current will not allow for center bias, resulting in asymmetrical clipping.
With a (transformer-loaded) powerstage optimum Class A doesn't mean 100% PD and vice versa.

The amp should give almost 4 times higher power with a 2 Ohm load.

I thought for a long time, that the goal for single ended was to bias at 100% plate dissipation. I have realised over time that this is not necessary or that it always sounds good.
This is especially true if the current available from the power supply is ‘unknown’ / reused from old equipment.

In a different thread you suggest that centre bias may not be desirable for guitar amps
https://music-electronics-forum.com/...lass-a-biasing

You're not alone.

But fact is that Leo's circuits work, stood the test of time and typically sound great.

I'm not sure that center biasing and symmetrical clipping is desirable in a guitar amp.

Any ‘rule of thumb’ applicable to a.) clean /max head room vs b) overdriven amps.
Yes by all means ‘trust your ears’ but some educated starting points are worthwhile, especially with less common output tubes and OT loads.

[Helmholtz]

This thread here is about maximizing power with SE class A and given B+.

Close to center biasing gives max. clean output.
Always a good idea to stay close to tube datasheet examples.

Even center-bias does not really give a symmetrical waveform with SE clipping, because one side clips by cut-off and the other by running out of plate voltage.

With the amp here, power output will stay below 1W unless the OT is changed.