Supro Guitar Amp

going downtown with a Supro/Valco/Wards Airline/we don'tknnow who made it, type of amp

we got 3 inputs, and two knobs,  which are not original but came outof the junk drawer,

guts

more guts

shielded input jacks because this is a grid leaker,

had original 6973 pwr tubes and 12AX7a RCA's also,

schematic,,

screens are running above B+ due to OPT DCR,

voltages in red are AC,

No output tube cathode bias cap?

> No output tube cathode bias cap?

Push-pull.

The electronic difference is small. The cost of the cap was a budget breaker.

You want refinement, don't buy Kent/Supro/Valco/Wards Airline.

no lytics on the preamp tubes either,

amp breaks up at half volume already so no more gain required,

nothing to dry out except the B+ caps and they are canned tuna,

I have a question on that schematic, V1 cathode pin3 is grounded, and the grid is grounded through a 6.8M resistor, so where does the bias come from?

Thanks,
Gene

Isn't 100 K (47K+47K) an unusual low input impedance for a guitar amplifier?
(Most of the time I see 1 Meg or even higher.)

Looks like contact bias input and Paraphase output.  Paraphase outputs look like a fun way to maybe get a different output section distortion.  One could make part of the voltage divider to the inverting amp section adjustable. 

I would guess the series 47K + 47K is an input voltage divider to lower the input voltage and way to tame the high end from the guitar due to loading.

I guess the lack of caps could be cost or enough gain without them as others PRR and CJ posted

http://www.w8ji.com/fusing_and_floating_grids.htm

> where does the bias come from?

Don't put any grid-leak on a tube. For small types with significant external plate resistor, the grid will tend to drift a part-volt negative and the operating conditions suitable for small-signal work. With infinite resistance it is not real stable, and a strong transient can throw it so far off that it quits working. Values like 1Meg are uttlerly stable and essentially zero bias. 10Meg is a workable "contact bias" for most of the small tubes. 6.8Meg may have been on sale that week.

While there are contact-potential effects, this leak-bias is more about the spread of initial energies of the cathode electrons. Some will jump into the grid even when it is a little negative and make it more negative. Too negative, they stop going there. In healthy tubes you get a few tenths volt negative bias.

This is OK for dynamic mikes and early e-guitars. As hotter sources came out, most G-amps went to cathode bias. This amp's divider input also takes somewhat higher inputs, at the cost of S/N. If you care, buy something else.

this type of bias can sound great due to a sort of compressed sound that you get,

drawbacks are that you do not want to take it out of your living room lest you want to pick up every radio station or light dimmer in town,

also, it does not like stomp boxes very much due to the low headroom,

also of interest in this amp is the ,05 cap coming off the 220K  plate resistor in stage one, some type of tone shaping or stability going on here,

> the ,05 cap coming off the 220K  plate resistor in stage one, some type of tone shaping or stability going on here

0.05uFd + 100K is 33Hz.

It isn't guitar-band bass-boosting: too low and too small to hear.

It is the "B+ filter cap" for that stage. 33Hz will knock-down 120Hz ripple some; it also reduces bass sneak-back through the B+ chain from higher level stages.

With more money we might use 10uFd here to get for-sure ripple knock-down and better subsonic stability.

But it is what it was. You can put a touchscreen infotainment/GPS system in your model T, but that's just wrong.

PRR said:

Don't put any grid-leak on a tube.

Click to expand...

I was not aware of this biasing method. I guess I was thinking that any stray electrons that landed on a grid would only amount to something like a tiny random static charge, not enough to be useful, but needed to be drained off with the usual 1meg for stability. My assumption was that the grid and cathode both at ground potential means non-linear, like a detector in an AM radio.

Gus mentioned "contact bias", so I googled and read, eventually coming full circle back here, to this oldie but goodie:

http://groupdiy.com/index.php?topic=7623.0

Thanks all, I just learned a bunch. 

Gene

Gus said:

No output tube cathode bias cap?

Click to expand...

As luck would have it, without that cap even sounds 'better',
as described here:

https://www.premierguitar.com/articles/26171-ask-amp-man-modding-a-supro-6420-thunderbolt

This might be a good mod for bass, but it certainly lost a bit of guitar magic. No bypass cap for you, Thunderbolt!

Click to expand...

Bye

cool article, thanks!