Supro 1624T - fixed bias?

[adamG]

Hi Guys,

Could you advice me how to mod this circuit into fixed bias?
My intention is to adjust the power stage plates on 400VDC, 2nd grids 290VDC. So, in that way, I'd like to receive 24W.
Do you think it's a good idea?
I've noticed that few US companies do such projects.

Thank you in advance.

Regards,

Adam

[sluckey]

This pic comes from the RCA RC-30 Tube Manual. I don't care for the bias supply. I'd just copy the Princeton Reverb bias supply instead...

[PRR]

> adjust the power stage plates on 400VDC

How are you going to do that?

If you don't raise the B+ much, the difference in Power is insignificant.

Anyway 17W to 24W is not a big increase. Just audible side-by-side. If 17W is not enough, 24W is not bliss.

[adamG]

Because of this:
https://suprousa.com/amplifiers/1624t-dual-tone/

and this:
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/6/6973.pdf

Do you think it is a kind idiot idea?
May it is. If so, then please correct me.

Thank you in advance.

Regards,

Adam

[adamG]

I don't care for the bias supply. I'd just copy the Princeton Reverb bias supply instead...

Steve,

Thank you very much!

So, if I adopt Princeton bias ,should I supply negative bias voltage to the point between 12K and 470K resistors?
Also, I like MV very much. For me is very tempting to provide it instead of two 270K resistors.
Do you think it does make sense? I assume 500kA pot. 

Regards,

Adam

[sluckey]

Oops! I forgot about that old paraphase inverter. That complicates converting to fixed bias. You would need to add two more coupling caps just prior to the output tube grids to isolate that voltage divider for the paraphase inverter. And then add two 470K grid leak resistors after the caps. The fixed bias would connect to the junction of those two resistors.

You did notice that the reissue 1624T is NOT the same as the original 1624? The reissue has four 12AX7s rather than three. And it has two tone knobs rather than one. And the input jacks are quite different too. I'd like to see a schematic but that's probably not gonna happen. No telling what the differences really are. Could be the only similarity is the model number? 

If you want to clone the reissue amp you may want to find a schematic rather than guess at the circuit. I'm currently building an old S-6424 largely because of a demo I heard of the reissue amp but also because of my curiosity about the 6973 tubes. I hope it sounds good but just don't really know what to expect. I didn't care for Supro back in the '60s. They were a low end amp and not very popular. If not for Jimmy Page the name Supro would have probably just slipped into oblivion.

[DummyLoad]

if you want more power, consider using a quad of 6973. should be near or same tone with twice the power - select an OT with half the primary impedance (4k-5k-ish) and PT with the similar HV sec. voltage rating with near to double or double the current output (hammond 272DX??), use a 5U4GB tube rectifier. should net about 35 watts + you'll need a speaker that can handle more than 25 watts.


i think the mojo in that amp is mostly in the paraphase and the cheap parts: wimpy OT, ceramic coupling caps, carbon comp res., low budget 1/2" plywood cabinet but finished off with a relatively decent jensen ceramic mag. speaker. 


--pete

[adamG]

Thank you very much!
To tell you the truth, I did forgot the solution of 4 power tubes. Silly me 
That idea seems to be very resonably and simple.

Regards,

Adam

[adamG]

You did notice that the reissue 1624T is NOT the same as the original 1624? The reissue has four 12AX7s rather than three. And it has two tone knobs rather than one. And the input jacks are quite different too. I'd like to see a schematic but that's probably not gonna happen. No telling what the differences really are. Could be the only similarity is the model number? 

If you want to clone the reissue amp you may want to find a schematic rather than guess at the circuit.

Steve,

I didn't know about 1624 reissue with four 12AX7 tubes.
I just compared two att.schematics of 1624 and 6424.
They're quite similar to each other. Don't you think?
That is why, I though to myself that 6424 is a kind of modification/extension of 1624 with T 
Could you consider MV for 6424?

Regards,

Adam

[sluckey]

The 6424 and 1624 are the same except the 6424 has two tone knobs.

Could you consider MV for 6424?

I'd do it like this. See pic. And if you want to do fixed bias just remove the ground connection from the MV pot and connect the fixed bias at the junction of the MV pots.

[adamG]

Thank you Steve very much!

Regards,

Adam

[adamG]

Finally, I followed Sluckey's project of 6424.
My replica is with 4 inputs and three LEDs/two buttons FS.
Speaker is Jensen P12Q. The amp is much more treble and really cool now:)
If I'm asked to build another one, I'll build it with polyester caps to verify a tonal difference.
Here are some pics...

Regards,

Adam.

[tubenit]

That is a very beautiful amp!  Well done!  Thank you for sharing the photos. Outstanding.

With respect, Tubenit

[ClippIn4tae]

My goodness thats magnificent!
Ive been wanting to use the series electrolytic filter caps on my next build. For alot of reasons and i was curious of how you attached them to the board?  Solder from beneath?  My my thats the finest build i think ive ever seen! How are you enjoying the amp? How did the cathode bias treat you?ive been curious to how loud the 17/24watts would be?  Im just in awe,gorgeous!!!

I would love any and all info in your build strategy.

Beautiful!!!!

Im gonna be doing a similar one this week.
My my....

[sluckey]

That's a very nice looking amp Adam, inside and out. Faceplate really looks sharp.

[kagliostro]

 


Franco

[adamG]

Ive been wanting to use the series electrolytic filter caps on my next build. For alot of reasons and i was curious of how you attached them to the board?  Solder from beneath?  My my thats the finest build i think ive ever seen! How are you enjoying the amp? How did the cathode bias treat you?ive been curious to how loud the 17/24watts would be?  Im just in awe,gorgeous!!!

Thank you very much:)
Yes, you're right. I soldier them from beneath. I just drill 4 holes for each. Middle ones to fast and soldier a cap. Outers for its leads. 
I prefer can electrolytic caps because of three reasons.
First, the voltage durability of a pair of them. Second, they're much cheaper:). Third, availability.
My replica has up to 23W with 352V on plates and 360R resistor.
Switch Half/Full as a normal DPDT toggle and two screen resistor. Each of 2W.
Yes, that combo is not my favourite tonal territory, but I am proud of this build. My friend the owner of it is rather very happy.
I took me approx. 6 weeks to build it.

Regards,

Adam

[adamG]

That's a very nice looking amp Adam, inside and out. Faceplate really looks sharp.

Thank you Steve very much! Especially to you, because I was able to make it because of your generosity, your project!

Best regards,

Adam

[adamG]

Thank you Franco. I can see that you're so kind looking at my job fruits from time to time:)

Regards,

Adam

[adamG]

Forgive me for another pics...

I plan to accomodate my next buil TW Express in same chassis and combo:)
I did not know how beautiful of that dims combo is. It is very handy and good looking! Just a small, happy baby:)

Regards,

Adam

[ClippIn4tae]

ah ha!i knew it.

And among alot of guys,like the valve wizard, agree about the series smoothing caps.

Fantasic ,also thanks a million for saving myself a chassis build and then gutting.

I have been in a pickle about a circuit i am gonna build this coming build and its not gonna be a 6161/6424/1624.

But im gonna use these pro com 6973s in a ab763ish build.

I just love the leo forgiveness/freedom of expression that comes from that style .

390r cathode resistors huh?@23watts?

I got some 270r handy, i will get some larger i would like to bypass @20 or so.

Thanks,my goodness im still blown away.

You sir have yet again raised the bar.

AMAZING, do you have any further info out there "mr.vintage"?

[Willabe]

                    ~Really~ nice looking amp!

Where did you get the cab, knobs and face/back plates?

[adamG]

Where did you get the cab, knobs and face/back plates?

Thank you very much:)
This combo is a fruit of my hands. I attached a pics of an aluminium chassis. First, cut off a plate, that painted...
Cab is also my job.
Front faceplate is laser engraved - custom shop acc.to my project/idea.
There's no back one. Just Dymo printer labels:)
Knobs are easily available in my place, fortunately.
It's pity that I'm not a musician. Otherwise, I'll publish some clips. I'm not that brave:) But, the amp is a good sounding one to my ears.


Regards,

Adam

[adamG]

And among alot of guys,like the valve wizard, agree about the series smoothing caps.

Fantasic ,also thanks a million for saving myself a chassis build and then gutting.

I have been in a pickle about a circuit i am gonna build this coming build and its not gonna be a 6161/6424/1624.

But im gonna use these pro com 6973s in a ab763ish build.

I just love the leo forgiveness/freedom of expression that comes from that style .

390r cathode resistors huh?@23watts?

I got some 270r handy, i will get some larger i would like to bypass @20 or so.

Thanks,my goodness im still blown away.

You sir have yet again raised the bar.

AMAZING, do you have any further info out there "mr.vintage"?

Thank you very much:)
M.R. is a signature of someone very dear to me:). Vintage, because this is basis the old times amp, right?
:"...have yet again raised the bar...". It is a kind of mystery, when a man improves his skills without his consciousness...
I can recommend you polyester caps to be closer to characteristic Supro's grittiness. I've made some mistake with polyprophylene ones. These are too smooth for this circuit, IMO.
With my plate voltage approx.352V ,360R resistor gives me up to 23W. Look at this thread  http://el34world.com/Forum/index.php?topic=22149.100
I consulted with my friend to keep 23W. If something wrong is happening withing 6973 tubes, He'll change 360R to 390R or even higher one.

Regards,

Adam