What tube should go here? 1935 Oahu restoration

jthomas666

Member
Messages
920
Oh! I thought the schemo came from inside the amp. Sometimes (in the old amps) they are glued to the side of the cab or are in a little envelope. Ok... obviously that's different.

Looking at your photos and your schemo, it seems to me that the filter caps are missing. Now, someone with some real know-how should see if they agree with me on that. Your amp likely is one of the variants of the 510 series of Valco amps. See:

https://el34world.com/charts/Schematics/files/valco/Valco_Schematics.htm

I don't see your amp, specifically in this set of schematics, but you can compare some of the older amps with field coil speakers to your amp and get a sense of how they were wired.

This is pretty close to your amp, I think:
https://el34world.com/charts/Schematics/files/Valco/Valco_510_1b.pdf

Maybe even better:
https://elektrotanya.com/valco_supreme_510-1_sch.pdf/download.html

So, trace the pins of what we think is the cap can circuit ans see how they connect to the PT circuitry and the primary power nodes (e.g., the plates of the 6v6's) and compare that to the valco schematics. If I am right and the filter cap(s) were in the missing can, it really should be easy to figure out reasonable values. With 2 6v6s and a 5Y3 rectifier, you probably can use something like 16 mF at around 500 V. (again, somebody with more experience should jump in here)
 
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jthomas666

Member
Messages
920
Well I can seem to post a picture from Imgur. Grr. The Electrotanya schematic of the 510-1 shows the filter caps quite well and the plug that connects to the left socket and the speaker.
 
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6,477
I was wondering why there were not the required filter caps shown in that schematic. The shorter cap that is still there has three sections....and appears to need to be replaced. The picture that Jthomas provided us show that the larger cap, which is missing from your amp....has two sections...as best I can see.
 

jthomas666

Member
Messages
920
Hey Steppin' Wolf: Does the elektotanya schemo (i.e., the 510-1) seem to fit the OP's amp? And can you tell if the filter caps are all in the shorter cap can?

Hey OP: So if Steppin' Wolf says that the 510-1 schemo is accurate, now you can trace out the placement and values of the missing caps.
 
Messages
6,477
Hey Steppin' Wolf: Does the elektotanya schemo (i.e., the 510-1) seem to fit the OP's amp? And can you tell if the filter caps are all in the shorter cap can?

Hey OP: So if Steppin' Wolf says that the 510-1 schemo is accurate, now you can trace out the placement and values of the missing caps.

jthomas, I think OP is working with a single ended amp here...a single 6V6. I am accepting that Garrick’s drawing is correct except for the lack of the electrolytics. The listing that was linked for that Oahu for sale that has that larger cap shows that the can that is missing from gerrick’s amp contains the first two filter sections...two caps of 10mfd/450vdc ratings as I read them. The small can has a 40mfd/150vdc cap, a 40md/25vdc cap and a 20mfd/25vdc cap. Gerrick’s schematic shows these as the power tube cathode bypass cap, the driver cathode bypass and the input bypass respectively.
What will have to be done is to trace back from the plates of those three sections to understand the power supply to those plates from the caps in that larger can....the power rail has to be understood. I am confident that Gerrick can get that done. After all, he drew this schematic he shows us. Understanding how the power rail is laid out will also give an opportunity to double or triple check the scheme that has been draw.
fwiw, I viewed quite a number of Valvo schematics at EL34 World. This is is the closest to what gerrick is working that I could find. It does show how the power rail is set up...and that part may be very similar to what Gerrick has there. This circuit has three filters whereas the little 1x8 Oahu in that sale that has a similar physical layout to what Gerrick has sown us in his amp seems to have only two sections. That is accurate I think because there is a difference in how the screen grid voltage is found. Note the difference between what Gerrick has drawn and the top scheme here.....the Model 21.
https://el34world.com/charts/Schematics/files/Valco/Valco_national
_dobro_21_31.pdf
 

jthomas666

Member
Messages
920
Stupid me. (not being sarcastic here... I had it in my head that it had 2 6v6s) It goes to show that one's perception can be mislead by one's pre-conceptions. :oops: (i never use emojis, but I deserve one here)
 
Messages
305
Stupid me. (not being sarcastic here... I had it in my head that it had 2 6v6s) It goes to show that one's perception can be mislead by one's pre-conceptions. :oops: (i never use emojis, but I deserve one here)
At any rate Gerrick, it will take only two caps to establish the power rail unless I am mistaken.
So, I'm thinking that I will have to install a cap to ground at pin 3 and pin 5 of the cap can socket?
What I can't figure now is: why does the 5V supply connect to the cap can socket?
Bear in mind this schematic is untested, I did double check the wiring but the secondary leads from the PT are all deciphered through color coding and common sense (I hope).
The fact is that I got some momentum on this and then put it aside when I got stumped.
I'm thinking that I should try plugging it in w/o any tubes in place and measure a few voltages from the PT. Maybe, it will be easier to move forward then and solve the capacitor dilemma.
 
Messages
6,477
I could be wrong, but it seems that the plates are the key at this point....where do those wires lead.....they connect back to those two caps in the big can in s9me manner it seems to me.
 
Messages
305
So, the leads from the PT secondary that I marked as 5V are actually the high voltage leads, so the red and orange lead designations should be swapped.
I found about 5.5V on the rectifier filament supply and 652V AC on the PT secondary.
 
Messages
305
It looks like the 80 rectifier tube could handle these conditions.
I could be wrong, but it seems that the plates are the key at this point....where do those wires lead.....they connect back to those two caps in the big can in s9me manner it seems to me.
The cathode of the rectifier goes to one side of the field coil, the other side of the field coil is connected to pin 3 of the socket and then that leads to both sides of the OT and all the other plates in the circuit.
The plates of the rectifier tube form a loop at pin 6 with the PT.
 
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Messages
305
jthomas, I think OP is working with a single ended amp here...a single 6V6. I am accepting that Garrick’s drawing is correct except for the lack of the electrolytics. The listing that was linked for that Oahu for sale that has that larger cap shows that the can that is missing from gerrick’s amp contains the first two filter sections...two caps of 10mfd/450vdc ratings as I read them. The small can has a 40mfd/150vdc cap, a 40md/25vdc cap and a 20mfd/25vdc cap. Gerrick’s schematic shows these as the power tube cathode bypass cap, the driver cathode bypass and the input bypass respectively.
What will have to be done is to trace back from the plates of those three sections to understand the power supply to those plates from the caps in that larger can....the power rail has to be understood. I am confident that Gerrick can get that done. After all, he drew this schematic he shows us. Understanding how the power rail is laid out will also give an opportunity to double or triple check the scheme that has been draw.
fwiw, I viewed quite a number of Valvo schematics at EL34 World. This is is the closest to what gerrick is working that I could find. It does show how the power rail is set up...and that part may be very similar to what Gerrick has there. This circuit has three filters whereas the little 1x8 Oahu in that sale that has a similar physical layout to what Gerrick has sown us in his amp seems to have only two sections. That is accurate I think because there is a difference in how the screen grid voltage is found. Note the difference between what Gerrick has drawn and the top scheme here.....the Model 21.
https://el34world.com/charts/Schematics/files/Valco/Valco_national
_dobro_21_31.pdf
This schematic looks right, although, the field coil impedance is much higher in mine at 2500 ohms. I'm not sure if that would require a larger or smaller capacitor value for the power rail.
 

HotBluePlates

Member
Messages
13,894
It appears that it may be for a plug in cap can. ...
Here is the corrected schematic:

The existing multisection can capacitor is not drawn on your schematic.

I haven't attempted to trace wiring to re-draw your schematic (very hard to do with photos), but before we assume there is a missing plug-in cap, those existing capacitors need to be represented somewhere.
 




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