Silvertone 1485 - Output Transformer advice

[sonoflee]

Hello all,

I recently picked up 2 extremely beat up silvertone 1485 heads cheap from a guy who had done some ...interesting... mods. To refresh your memories, these are 4x6l6 amps that used two (wimpy) O.T.s, each running three 10" speakers (a 2.6 ohm load for each transformer) in a massive 6x10 cab. Schematic below.



Apparently, he had killed one or both of the OTs in both amps, and used some interesting methods to "fix" them.

In the first head, he took out both OTs and used a single (massive) OT from an old RCA theater amp. After a couple of replaced tubes, that seems to work fine, and the wiring pretty much makes sense to me.

The second head is more of a head scratcher, though. One of the OTs is fried, and he apparently hooked it up so that the remaining OT is running all of the output. This does not seem right to me, but I'm trying to figure out the best way to proceed... My goal is to be able to use the amp with one of my existing cabs (all 8 ohm, various configurations) if possible.

Option A: replace the dual OTs with a single, higher powered one - maybe a Fender twin OT? This is pretty much what he did with the first one, and I think it would work fine - except I'd have to make a new cabinet for the head, because I can't think of a high-enough powered transformer that would fit in the space vacated by the two smaller ones. If anybody has any ideas on one that might fit, please let me know!

Option B: Replace the one busted OT (or just do both) with the Silvertone-specific replacements from Weber or Mojo. These would fit the chassis, and have a better option in that they're tapped for 4/8 ohm, but I'd still be stuck with the issue of putting together a new cab (or at least of rewiring an existing one to handle the separate/dual 4 or 8 ohm loads). I was wondering if there could be a reasonable way to tie the dual OTs together to run into one cabinet? AND, if so, how would that change the output impedence (i.e., if I could tie the two OT's together at the 8 ohm tap, would that make it need a single 4 ohm load)?

Thanks for the knowledge!

[olddawg]

These amps are sought after by certain people, especially since Jack White started using them.. If you got them cheap and are not exactly what you want, I would put them on eBay as parts amps. You might get much more than you think and then be able to purchase something more conventional and skip the headache and expense.

[sonoflee]

These amps are sought after by certain people, especially since Jack White started using them.. If you got them cheap and are not exactly what you want, I would put them on eBay as parts amps. You might get much more than you think and then be able to purchase something more conventional and skip the headache and expense.

It looks like prices have cooled off significantly in the past few years, but I could still make a lot more than I paid... But I would still like to get the second one up and running to see what it sounds like. And if I can get it working with reasonably-sized cabinet, it might be exactly what I'm looking for, who knows?

[Steve Conner]

I think the only reason Silvertone used the twin OT arrangement was to use up a stock of OTs. A single 100W OT is really a better solution.

Tying two OTs in parallel doesn't change the impedance ratio. If each OT was originally meant to match two 6L6s to 3 ohms, then the parallel pair will still match two 6L6s to 3 ohms. If you then add another pair of tubes, the appropriate load is 1.5 ohms.

So, let's assume you take option B and tie the two OT secondaries in parallel. The 4 ohm tap will want a 2 ohm load and the 8 ohm tap will want 4 ohms.

Another alternative would be to connect the primaries of the two OTs in parallel and the two 4 ohm secondaries in series. This would normally match a 16 ohm load, but the extra pair of tubes drags it down to 8 ohms.

[RedHouse]

Consider running 4x 6V6's and just using the single OT that is left, you get a nice 40-50W amp and it remains "nearly" stock.
(4x 6V6's run very well on an OT meant for 2x 6L6's)

On the proverbial "other hand" you can do a wild re-work of the amp into something else, this a a 1484 I re-worked the tremolo channel into a Marshal'esque setup, changed the PI to a long tailed pair, and changed the power supply to a standard 2204 type (except the clean channel), changed it to 6V6 tubes, added a effects loop, and now the guy has a clean-channel silvertone sound, about 15W, and a overdrive Marshall sound in one amp.



The lower power output and PS draw mean the unit doesn't work so hard off that silly voltage-doubler PS.

[soundmasterg]

If you are going to use 6V6's I'd suggest to use JJ's for their higher voltage handling....the 1484 and 1485 have upwards of 450v B+ which most 6v6's can't handle.

Greg

[RedHouse]

If you are going to use 6V6's I'd suggest to use JJ's for their higher voltage handling....the 1484 and 1485 have upwards of 450v B+ which most 6v6's can't handle.

Greg

That's what the schematic says but in reality it wasn't that high, IIRC it was more like 413V on the plates, ...but... never-the-less, your right about the JJ's (which is what I used) and it works fine.

Besides who in their right mind would install NOS 6V6's in a mod'ed Silvertone? yeah, thought not.

[loudthud]

Doesn't that amp have two voltage doublers in series for the B+? A little creative re-wiring could reduce the B+ to 3/4 the stock value. Simply change one of the voltage doublers to a full wave bridge (to keep a somewhat balanced current in the PT secondary).

[Tom Phillips]

Doesn't that amp have two voltage doublers in series for the B+? ...

The two output transformers really catch your attention and I knew the amp used a voltage doubler but I never noticed that it was two voltage doublers in series. Someone must have really been struggling to use up available parts. The circuit does provide a 3/4 B+ screen supply.

[soundmasterg]

That's what the schematic says but in reality it wasn't that high, IIRC it was more like 413V on the plates, ...but... never-the-less, your right about the JJ's (which is what I used) and it works fine.

Besides who in their right mind would install NOS 6V6's in a mod'ed Silvertone? yeah, thought not.

The four 1484's and three 1485's that I've seen have all had B+ 450V or higher, with my brother's 1484 at 456V and the 1485's usually more like 460-470V. I've never seen one anywhere near 413V but maybe the one you saw had bad e-lytics? They are guaranteed for one year though so who knows!

Doesn't that amp have two voltage doublers in series for the B+? A little creative re-wiring could reduce the B+ to 3/4 the stock value. Simply change one of the voltage doublers to a full wave bridge (to keep a somewhat balanced current in the PT secondary).

Yes these amps do have two stacked doublers....the 3rd level would be about 340V B+ which 6V6's could certainly handle. A stock 1484 is only 25 watts RMS though, so a 1485 with all 6V6's running off 340V wouldn't give you a lot of power. There's the transformer mismatch too to consider.

Greg