Silvertone 1484 Restoration - almost there, but need assistance!

[q9522678]

I have moved the 68k resistor (R58) to its rightful home between the 22uf and 10uf power supply caps and all voltages are looking pretty good now. Luckily that was an easy fix.

Now looking at the reverb - will do some experimenting in terms of where to take the input. Have already moved the depth control to where R40 was on the recovery side of the reverb. Seems junction of R9, C9 and R15 produces a very strong signal and drives the tank well. This appears to be post volume and tone stack. From a a circuitry standpoint are there any issues taking the reverb source audio input from here? I noodled around for a bit in this configuration and no magic smoke was emitted from the amp. And I’m plugging it to pin 7 of V4 - where the Reverb depth used to be connected. So it’s fed in it entirety through the V5 drive circuit.

Will also try some different values for bypassing R42.. I’m not overly fussy with the reverb tank in terms of sound. Some would say it couldn’t be much worse than the piezo reverb - but so long as it sounds ok I’m happy. Otherwise I would’ve gutted it and installed a transformer driven fender style circuit. But being lazy I wanted to try and work with what was in there in terms of tube complement. Which looking at some Ampeg schematics is doable. And appears others have done it, but not much of a write up online about functional circuits. Once I get this working satisfactorily I’ll amend the schematic and post for others to use for others who wish to take the “simple” route for replacing the stock reverb (although it could’ve been easier to use the existing circuitry in the reverb as an effects loop and simply install digital reverb)...

Also thanks to everyone for the prompt and detailed response to date. Certainly appreciated. Whilst I know the basics of tube amps (from previous restorations and builds over the years, and restoring a number of old organs) my electronics knowledge is elementary at best...

. .... more to follow.

[q9522678]

Hi Harold,

Absolutely agree. I’m not necessarily trying to make this amp sound like the previous 1484s I played. When I finally got it working and fired up, I was surprised to find it sounded a little different. Not necessarily in a bad way as it sounds quite good. But yes, certainly understand tube amps, even from same model and year can vary quite substantially in terms of tone.

Some years ago I had essentially modded my different amps to sound very similar (Bluesbreaker, Bassman, Plexi, Deluxe, JCM 2000... ) - from chasing the sound I had in my head - through replacement of various components. It wasn’t my intent to do this, just happened that way. Now I’ve restored them all back to stock as I find it better to have different character amps as opposed to different model amps that all sound alike. That being said - was good experience in learning the innards of those amps..

Now that the voltages are looking good and I’m satisfied the amp is functioning as it should, I don’t plan to do anything to change the tone as it sounds pretty good. Am sure I could turn it into a fire breathing beast, but I already have a few of those!!

Thanks for your thoughts!

[Enzo]

As I look at the 1484 drawing, I see the reverb coming off the treble pot and the TOP of the volume, not AFTER the volume (which would be the wiper). R9 feeds the tone stack. I think you will find however that when you play into the amp this way, turn volume to zero, and the reverb continues to play through. By drawing reverb signal AFTER the volume control, the reverb level follows the volume level. R9 node is PRE tone stack, not post. It will not harm the amp this way, of course, but to me at least, having to adjust reverb whenever I adjust volume is, well, real inconvenient.

Nothing could be worse than the original piezo reverb. The screen door spring on the back of my house would sound better, I am convinced.

g1, He altered the original to be parallel, the original was push pull with almost the same circuit. SImilar, not same.

[q9522678]

Ah yes - MIs-read ye olde schematic. And yes - reverb does play through with volume down - ala original.

Where would be a good point post volume? Happy to try it.

Having played it a bit more, the reverb doesn’t sound too bad. I’m pleasantly surprised...

[Enzo]

Tack the wire right to the volume wiper, see how that works.

[q9522678]

Some how I knew you were going to say that! Being lazy was avoiding having to do a long shielded cable run to the volume - will grab the alligator clips and give it a test first. Just so happened the aforementioned spot at R9 is in an easy to get to location out of the rats nest in there so was easy to wire in.

But do like the idea of the reverb adjusting with the volume do may be a necessary evil...

[q9522678]

So tried going straight from the volume wiper. Didn’t seem to like it. Was very noisy (possibly due alligator clips going past tubes and PT but tried to get as much distant between them as I could. I persisted in order to see how an audio signal would sound from there driving the reverb and didn’t seem to get much at all? I toggled between the two locations and R9 was quieter and louder than direct off the volume pot. I honestly couldn’t get a whole lot from the volume pot unfortunately. Are there any other locations, post volume, that might be worth trying.

I note when going from R9 at around the “9” position on the reverb control the sound goes fully saturated. At around “9.5” the volume drops out and all that is left is the reverb signal. Guessing this is due to something being exceeded in the tank. Not a biggie as it is at the very last part of travel of the reverb knob, but open to suggestions on what this might be and how to fix.

Reference comments above on cap bypass for R42 - what sort of value would be recommended here as a starting point?

[Helmholtz]

Is the reverb transformer still the original one? If it was used in a PP arrangement driving a high impedance piezo transducer, changes are that high that it won't work (properly) with an AB763 reverb driver and inductive transducer. There is impedance matching and power transfer involved like with an OT.

Could you post the original circuit?

Reference comments above on cap bypass for R42 - what sort of value would be recommended here as a starting point?

Try 2.2µ/63V with R42=10k.

[Enzo]

There is no transformer in the original. The two plates are out of phase and go through caps to either end of a piezo flake.

http://www.prowessamplifiers.com/sch...ertone1484.pdf

[Helmholtz]

There is no transformer in the original. The two plates are out of phase and go through caps to either end of a piezo flake.

http://www.prowessamplifiers.com/sch...ertone1484.pdf

Oh, thanks. The transformerless PP circuit makes a lot more sense with the high voltage, high impedance capacitive piezo transducer load. So I assume the reverb transformer has been chosen to match the intended tank.

As always, the usefullness of (my) replies depends on the information provided.

[q9522678]

Hi Helmholtz,

As Enzo said, the original reverb circuit is a tube driven transformerless reverb. Here is w redrawn schematic of the entire 1484 circuit:

https://www.dropbox.com/s/segd5dpxbr...%2009.gif?dl=0

I’m replacing the original reverb with a high impedance tube driven transformerless reverb using the existing reverb tube compliment and as much as the original circuitry as possible for minimal overhead. The original reverbs in these often aren’t working and if they are, don’t sound particularly good (although I appreciate that’s a matter of taste).

The new tank has 800 ohm input impedance and 2250 output impedance - is a short three spring tank that I plan to mount to the head shell about the preamp and reverb tubes - have already test fit and it works well.

The reverb is now functioning and sounding pretty good. Just at the tweaking phase now. The two outstanding questions are:

- value of the recommended bypas cap. I see you’ve posted 2.2uf /25v - do will give that a shot.
- when the reverb pot is turned full on the main input drops out and can only hear the reverb treated audio. This only occurs on full. At around 9.5 on the pot there is full lush audio and reverb signal. Not a big deal but curious if anyone has suggestions as to why and what might be done to rectify.

Also open to any mix suggestions, I changed the coupling caps to 0.22 (from 0.022) and sounds quite good.

Again - appreciate all of the feedback to date. Thankyou

[Helmholtz]

https://www.dropbox.com/s/segd5dpxbr...%2009.gif?dl=0

This circuit is dubious and certainly contains errors, at least it is incomplete.

I’m replacing the original reverb with a high impedance tube driven transformerless reverb...

Maybe I misunderstand (not being a native english speaker), but I thought you are replacing a transformerless reverb with a transformer coupled reverb?

- when the reverb pot is turned full on the main input drops out and can only hear the reverb treated audio.

Might indicate grid conduction/overload of V4a.

Also open to any mix suggestions, I changed the coupling caps to 0.22 (from 0.022) and sounds quite good.

If you rewire the reverb driver to Fender AB763 specs, you don't need coupling caps and plate resistors but will get more reverb drive.
What are the specs of your reverb transformer?

[q9522678]

Hi Helmholtz

Your English seems better than mine!

Yes - would seem the original schematic that someone drew up for a tranformless reverb was somewhat incomplete. No matter - as working through it now. Once I’m done I’ll update and upload it for others to use.

I’m not using a transformer. My approach to this built was as little work and fewer parts as possible. So using the existing tubes and no transformer.

Thanks for the advice on the audio dropping out at full turn. Perhaps requires a resistor to drop the input?

[Helmholtz]

I’m not using a transformer. My approach to this built was as little work and fewer parts as possible. So using the existing tubes and no transformer.

Oh, I see. Seems I was on the wrong track since a while. So transformerless current drive. Good approach nevertheless! Mind that the coupling cap(s) become part of a series resonant circuit together with the transducer inductance and thus impact reverb frequency response. (You might still directly connect V5 plates and use a single common 15k plate resistor and a single coupling capacitor - won't change much, though.)

Thanks for the advice on the audio dropping out at full turn. Perhaps requires a resistor to drop the input?

You might try a 100k "grid stopper".


If you want more reverb treble, wire a 0.002µ (2nF) cap across the 220k grid resistor (i.e. between grid and ground) of V4b.

[q9522678]

Apologies if I wasn’t clear, but certainly appreciate the input.

When I get a chance I’ll try the recommended caps and the grid stopper and report back. Almost there!!