Other Amps

1432 pre-amp tubes

1

Howdy all, this is my first time back since the crash….anyway….I’m looking for some tech advice. I’ve been playing through my silvertone 1432 a lot lately, and swapping preamp tubes quite a bit. I’m thinking of trying a 12DW7 in place of the 12ax7. Going for a lower gain on one input, higher on the other, to a/b inputs. I was hoping someone with some tech chops could have a look at the schematic(link below), and let me know if that’ll hurt anything based on the circuitry. Also, because it’s a split-chassis amp, there is a phono plug that connects the preamp to the power section. Could I use that connection as an effects send/return and place a OD/echo/reverb pedal in between without negatively affecting the performance of the amp? Inquiring minds want to know….

Schematic link:

Thanks for the help!

2

The same tube is used for both the instrument inputs and the mic input. It won't matter what you change it to. The preamp out level is too strong for the input of a pedal, so that idea won't work either.

3

OK., I understand that the preamp out level could be too hot. But..isn't the 12ax7 preamp tube being split between Inst. and mic input? half & half? because then they go to separate tone/volume controls....I a/b mic/inst channel all the time with different tone/volumes, but was trying to take advantage of the different sides of the 12DW7 for a gain difference on different inputs..... My big concern was ensuring that it doesn't cause any voltage problems if I pop a DW7 in there...

4

Both sides of the tube have the same gain. Swaps for 12AX7 would be a 12AT7 or a 12AY7, both with less gain.

5

A 12DW7 has different gains. one half is a 12ax7 and other half a 12au7, that's why I want to use it, but I'm concerned if this affects anything down the line, with voltage, draw, etc., on this old amp.....

6

Right. That would probably be fine as the 12AU7 is similar to the 12AX7, but just lower gain. The way my luck works, the hotter side would be on the wrong input.

7

I don't know about your amp, but that works on a Marshall DSL 15.

8

Cool. The dsl is also 2 x 6v6, but it has a few more preamp tubes! Where do you use the 12DW7? and why? I can't imagine wanting to tame the gain on that amp...that's what its about, innit?

9

It's in V1. And although my tech very clearly told me he was using a 12DW7, the tube that is in there is very clearly marked RCA 12BH7A. That is not like a 12DW7, so I'm not sure what he did...it sounds great. It is quiet and it breaks up beautifully.

The gain in the drive channel was crazy fizzy over the top noisy, and the clean channel is clean clean clean. The explanation is that on a DSL 15 they used Red 2 gain rather than Red 1 gain structure. So as it came, there was a big gap in the blues rock/classic rock zone. The 80% gain reduction on that tube takes me so the drive channel starts in bluesy cleans and ends up in the Metallica area. If I need more, pedals.

Looking up the 12BH7A, it looks like it's similar to a 12AU7 only more headroom and it draws more current. So I wonder what Russ did to accommodate that and how he kept the clean channel solid? Magic, I guess.

10

A 12AU7 provides very weak gain for the first stage of a guitar amp. Major tone suck.

I usually can't go lower than a 12AY7 or 12AV7 in terms of gain.

11

That what I'm concerned about, the current draw. The amp sounds great as is, but it is old and I don't want to alter any of the guts or put unnecessary strain on any other components to have the a/b option. I was hoping someone would chime in that understands the schematic and explain why its ok or not on that amp to drop the dw7 in there. Sounds like that dsl 15 can cover a lot of tone territory. That's just a head, right? What are you using for a cabinet? I'd love to find someone that can build a clone of my 1432 cabinet to really bring it to life...

12

A 12AU7 provides very weak gain for the first stage of a guitar amp. Major tone suck.

I usually can't go lower than a 12AY7 or 12AV7 in terms of gain.

– guitarcapo

Is there a different dual triode tube to try that has 12ax7 on one side and an AY or AT on the other?

13

I have a couple cabinets I use with the DSL15. I have a slant front Marshall Haze cabinet with a Celestion Marquee and an old Peavey 2x12 cab with a Weber Blue Dog/Silver Bell combination. I think those Haze 1x12 cabinets are a steal, I love the tone. Of course your mileage may vary, I see some folks can't swap the speaker out fast enough. I'm playing Gretsches with Filters and single coil Tele's through them...so I'm not hitting that preamp section very hard, either.

14

A 12BH7A has a gain of 17 on each triode. A 12DW7 has a gain of 17 on the first triode(pins 1, 2, 3) and a gain of 100 on the second triode( pins 6, 7, 8)

That being said, it means the microphone channel will have the gain factor of 100, the instrument channel 17.

You can plug and play.

A 12AX7 has a gain factor of 100, a 5751 or a 12AT7 60, a 12AY7 40 and a 12AU7 17.

You can safely pull one and plug another in

15

Cool...Thanks man, much appreciated.


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