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

[waspclothes]

I came across this little RCE amplifier made in Canada, model G-5. Presumably a 5W amplifier (?) that is only putting out about 0.6W into my 8ohm dummy load.

Attached is the schematic with voltage measurements. Everything seems fine up until the output tube, but swapping that out with a new tube yielded no additional volume. I also replaced the .002uF capacitor that ties the 6V6 Plate to B2, although I have no clue what the purpose of that capacitor is. I also tried a new rectifier tube, wondering maybe all of the amp's voltages were low.. but everything stayed the same.

Seems like it could be a cool little amp.

I've attached a schematic with the voltage measurements and the amplitude of the input signal in different stages.

[glebert]

Are you able to measure the AC voltages at the plate of the 6V6? What is it like with a 4 ohm load since that is what the schematic calls for?

I have a Magnatone 410 (single 6V6) with 4 ohm speaker (and additional speaker out?) that I have never been able to get over 2.5 watts.

[Helmholtz]

Where's the 5W number coming from?

I think your output measurements make sense.
Max. theoretical output is around 0.9W into 8 Ohm with the data provided.

Problem is the large OT primary impedance.
With a turns ratio of 45:1 the primary impedance with an 8R load is 16.2k.
Optimum primary impedance would be around 4k, which should give you up to 3W output (minus OT losses).

Look at the design examples in the datasheet (Class A, Vp=180V and Vp=250V):
https://frank.pocnet.net/sheets/191/6/6V6.pdf

[waspclothes]

Hi Helmholtz,

The 5W was just a blind guess based on the model name, but also the expected output from a Tweed Champ or some sort of similar amp. Is it just that the Tweed Champ uses a more efficient OT?

[Helmholtz]

Hi Helmholtz,

The 5W was just a blind guess based on the model name, but also the expected output from a Tweed Champ or some sort of similar amp. Is it just that the Tweed Champ uses a more efficient OT?

Main difference is higher plate and screen voltages followed by OT impedance.

[g1]

Seems to me the idle current is fairly low for single ended 6V6.
I'm wondering if the B+ is up to snuff? Have you tried another rectifier and checked for ripple on B+ ?

[mozz]

Yes, check the power supply resistors.

[Helmholtz]

Seems to me the idle current is fairly low for single ended 6V6.

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.

[waspclothes]

I did test the amp with the internal 4 ohm speaker and the output was also very quiet and less than 1W.

I also tried a new rectifier tube, same voltages.

The power resistors with my multimeter and they were all pretty close to spec.


Oh wow, I just saw the header message about Enzo. Super sad about this.. I'm one of the thousands that probably benefited from Enzo's time and insight... that sucks.

[Helmholtz]

What are the HT (PT secondary) AC voltages (loaded)?

Do you measure output power using a scope to see onset of clipping?

[g1]

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.

Ok, I can accept that, although I am used to seeing asymmetrical clipping and often more than 100% PD with single ended 6V6's etc.

What do you think of 236V B+ off 240-0-240 HT winding? And that is an old 25Hz 115V transformer so I would think it will be putting out even more than the 240-0-240.
(see pg.14 of attached)

[waspclothes]

What are the HT (PT secondary) AC voltages (loaded)?

Do you measure output power using a scope to see onset of clipping?

Yes, I used a scope and that measurement was before clipping.

I didn't measure the voltages coming off the PT, sorry. Next time I have the amp open on the bench, I'll check that out... I kinda just bolted it back up and tucked it away to forget about after being disappointed in it's low volume.

[Helmholtz]

Ok, I can accept that, although I am used to seeing asymmetrical clipping and often more than 100% PD with single ended 6V6's etc.

No contradiction to what I said above.

When a SE class A stage clips asymmetrically, it's not center-biased. One might call this class AB, if that's even defined for SE.

Higher B+ forces higher idle current for center bias (i.e. with same Raa), often leading to excessive PD.
It all gets clear from loadline construction.

[Helmholtz]

. I kinda just bolted it back up and tucked it away to forget about after being disappointed in it's low volume.

Why not replace the OT with a slightly bigger 4k:8 Ohm type?

A cathode cap (say 50µ/25V) for the 6V6 would increase gain.

[Helmholtz]

What do you think of 236V B+ off 240-0-240 HT winding? And that is an old 25Hz 115V transformer so I would think it will be putting out even more than the 240-0-240.
(see pg.14 of attached)

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.
So that looks possible. The 5Y3 will drop a few volts as well.
Also consider that Class A means full load even at idle.

Where/when did you have a 25Hz grid?