Dukane 1A385

[angelodp]

Hi I have a Dukane 1A385 in good cosmetic shape. Beginning to scope it out for a conversion.

AC cord not grounded yet but works, 117 inside the amp. Fuse good, switch on/off good, bulb good.

I do not get that bulb lighting up, no 6.3v on heaters. Maybe the PT is kaput?

Next steps to check out?

A

[turbofeedus]

5V rectifier heater working? Any B+ in the amp?
Check for loose or poor solder connections starting from the lamp outward (assuming it's first in the chain)?
A 5E3 PT should work if this one is toast.

[Phil_S]

+1 on checking out the lamp. Occam's razor, the simplest explanation is often the right one.

These amps are built like tanks. I acquired one 4-5 years ago, a 1F460, from a retired service technician who put it on Craig's list. I haven't got around to the guitar amp conversion yet. It came with original USA glass, including a pair of 5881's, with very low hours of use. The seller said it was the spare/loaner provided to customers while he worked on their equipment. He said it's practically brand new.

[angelodp]

Yes, this one is a tank for sure and quite clean inside. The PT has some rust.... I don't get 5V or plates or anything?? I'll look at connections but it all looks original. The power input has a 117 or 127v wind. What would 127v be for.

A

[Stevem]

With the power cable unplugged from the wall what then do you read for resistance across the black/ red and black wire on the PTs primary, I bet it's open?

[Phil_S]

The 127V primary is in case the line voltage is high. It is a way of taming the higher secondary voltage that would result from using the 117V winding with 127V at the wall socket. I can't imagine when this amp was made that you'd have used anything but the 117 primary. Today, that might be the more appropriate selection.

FWIW, I am thinking Steve is on to something. The good news is that all the voltages are given and you know the tube compliment. Finding a replacement at a reasonable price should be relatively easy and not too hard on your wallet.

[angelodp]

Correction, 4.3 ohms black to black/red 4.7 ohms black to black yellow - my error.

??

[Phil_S]

That seems wrong to me. According to the schematic, Blk/Yel is the outside leg of the winding and Blk/Red is the inside leg. Blk/Red is shorter, therefore fewer ohms. I suppose it is possible that the lead extensions were not attached correctly.

Anyway, it appears there is continuity on the primary. Have you plugged this in and checked for voltage at the secondary hookups? If so, what did you find?

[angelodp]

Had my buddy Stevo by, excellent trouble shooter. Turns out the fuse holder was not correct. Had 117 on one side but not the other. Changed out the fuse holder, now working. Onto the recap and input and output. See/hear how it sounds and then the tweaks.

Thanks All.... Stevo especially

Ange

[angelodp]

Phil, I'll double check that value. Here is a revised schematic for this amp. I am looking for any suggestions on improving this.

1. 1.5k grid Stoppers on 6V6

2. Screen grid R's - 470R

3. NFB control

Cheers A

[Stevem]

Here's a suggestion, r9 and r19 need to be very closely matched so I would replace those with 1% tolerance resistors and since 12AU7s draw a lot of current R19 should also be a 1 watt resistor, in fact any 1/2 watt resistor in the amp with close to 300 volts or greater I would at some point change out for 1 watt resistors.

Metal film type resistors ( cathode and plate load ) are much quieter then carbon comp ones in terms of noise hiss, especially old carbon comp types in that amp.

Resistors in the location of R12 have been known to add a bunch of unessarry hiss to the signal path also.

I would up the wattage of R16 to 7 or 10 watts and keep it well away from C2, and in terms of C2 I would audition changing it to a 100 uf cap to see how you like it.

If there's room within the amp I would do away with the multisection filter cans and use seserate filters this would be especially helpful somyiu could get the ground for the PI sections plate filter to go over to the ground point of the first filter node where it should be.

I would also up that 5 uf filter to 16 uf.

[turbofeedus]

Never seen a cascode in the wild, only read about them on Merlin's site. Cool!
Might try one of those in a EL84 amp I'm building soon.

[Phil_S]

Angelo,
Glad you got it sorted out with an easy fix. I'm a huge fan of this type of project. Only your imagination will limit what you can do with it. Because all the basics are there, your out of pocket cost should be relatively low. I'll look forward to seeing what you do with it!
Phil

[angelodp]

Thanks Phil and Stevem, I will begin simply by hearing this circuit as is. Put in new cathode caps and a jack, IEC next and output jacks. Its a swimming pool in there

[angelodp]

Here's a potential roadmap.