Gulbransen APS 25 conversion

[Toxophilite]

Hello
I've gone back to tinkering with this Gulbransen APS-25 organ amp I've had for a long time
I only have a schematic for the power amp and PI but I added another 12AXY in front and I'm using one half of it with a fender AB763 tone stack and wiring. It works and sounds decent though there is a slight hum (even when the volume etc is right down)
I went through the grounds and wired them better but it's still there. So that's something I'm looking at


What really perplexes me is that I have 259vdc B+ going into the EL84s and about 356vdc at the plates!
THat doesn't right to me



Any ideas?

[kagliostro]

What really perplexes me is that I have 259vdc B+ going into the EL84s and about 356vdc at the plates!THat doesn't right to me

A wrong measure ?

259vdc B+ going into the EL84s

where on the el84 ?

K

[John]

Which pin are you measuring the 259 vdc at?

[Toxophilite]

PIn 9

[John]

Okay, this is just a guess. It might be that the e-cap at that 33k/1W is a little leaky, and it's draining a bit of voltage to ground, thereby lowering the screen voltage. And that might be the source of your hum, too. Since that's where you're also getting the B+ for the preamp tubes (according to your schematic), then the plates on the preamp tubes are probably running around 200'ish?


Sluckey will probably know exactly what it is, though.

[sluckey]

Nothing odd about it at all. Look at the schematic.

[Toxophilite]

 
This is where my lack of knowledge is really letting me down!
I'm still at the stage where I can read the schematic and hook up all the bits...and I even understand some of it!
But not all of it
If you care to explain I'd really appreciate it, but I can also try to do some more reading to save you're time


Knowing that it's not a problem is good!
Thanks

[sluckey]

The plates get their voltage thru the OT primary and from the first filter cap which is approx. 360V. (see red path)

That 33K/1W resistor drops that B+ down to 259V at the second filter cap. The screens get their voltage from this second cap. (see blue path)

Since there is approximately 100V dropped across that 33K resistor, there must be about 3mA flowing through that resistor and that's a believable amount of current.

[John]

I wasn't figuring on that much voltage drop over the resistor.


Do I get a C- anyway? 

[Toxophilite]

THanks so much
It's sad but true but I didn't know the power tube plates got their B+ from the OPT!

[sluckey]

I wasn't figuring on that much voltage drop over the resistor.

It's a BIGGG resistor. Doesn't take much current flowing thru it to cause a big voltage drop. We don't often see screens operated 100V below plates in small amps. But this wasn't meant to be a guitar amp. I would probably decrease the size of that resistor to ≈1K if I was looking to convert to an AB763.

[Toxophilite]

 
I actually have it at 10k right now and it's still around 250
I also decreased the one following(to the 12ax7 plates) to 4.7 K top get more juice to the preamp . It's about 150vdc now on the 1st
stage


The amp is quite loud when cranked but very clean all the way up with a bit of distortion around 9, just a bit though


More voltage to the EL84 screens would probably give me more volume overall, but likely no more clean volume
Is that right??

[shooter]

More voltage to the EL84 screens

I believe you're right, I think a lower voltage, or more stable voltage yields a cleaner PA.

[Toxophilite]

I have 1/2 a 12ax7 that's currently unused on this amp
I was thinking maybe tremelo
Any suggestions as to a 1/2 12ax7 tremelo circuit that will work with this power amp
Marshall 18 watt?


Or if someone has a better suggestion as to what to use the unused triode for

[sluckey]

Since you basically have a Princeton AA964 preamp, copy the Princeton AA964 tremolo circuit also. It's a nice sounding amp and has a good tremolo.

[Toxophilite]

It looks good and I can use the same bias steal



But wouldn't I need to change how the 'screens' and cathodes of the EL84s are wired?

[shooter]

change how the 'screens' and cathodes of the EL84s are wired?

To make it a aa964 I'd guess you would, don't know if you could inject it on the PI by spitting up the cathode resistor?

Where is the other 1/2 being used, I see 1 tube in the schematic, not 1-1/2

[sluckey]

You don't need to mess with the bias, or the screens, or the cathodes. Just make these simple changes.

EDIT... fixed error on schematic.

[Toxophilite]

Cool!
I'll give that a try
Thanks very much!


Shooter; I added another 12ax7 to this chassis so i could put a tone stack in and that leaves me with an extra 1/2 triode as i only needed one half, I should've added it to the schematic I posted

[shooter]

I should've added it to the schematic I posted

NBD, I just confuse easy 

Also thx, Steve, I have been wondering if you could do something like that with a cathode biased PA, now I know!!

[Kasanay]

Nothing odd about it at all. Look at the schematic.

I know this is an old thread, but the schematic is incorrect. I have a pair of these amps and I wanted to let any future readers know that the 33K resister shown in the schematic is really a 3.3K. I believe the “.” didnt print.

[PRR]

> really a 3.3K.

Welcome. Agree. EL84 G2 may suck over 10mA. In 33K that would drop 330V, which is about all we have, so about zero to G2, which can't be right. 3.3K seems likelier. Thanks.

[Kasanay]

Yes, this schematic really messed me up when I was overhauling one of mine. There are several copies of it floating around the inter-web and they all have the same error.
One of the organs that I picked up came with all the original paperwork - including a copy of this schematic and sure 'nuff, the error is even in the printed copy.

Mine are APS-22 (I  now have 3 of these). I have never been able to locate a schematic that was specifically for the APS-22, but , other that the 3.3K error, this schematic matches the '22 perfectly.

I've attached a few photos showing the 3.3K (Orange, Orange, Red) resistor.

[Kasanay]

Here is the APS-25 (APS-22) schematic.