Silvertone 1484 Restoration - almost there, but need assistance!

[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]

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.

[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]

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.

[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?

[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...

[Enzo]

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

[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]

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]

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!

[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.

[HaroldBrooks]

Just a general statement about the same amp having a different level of gain... I have amps with very close circuits, and sometimes the gain is radically different from amp to amp and it's usually a big combination of things, not just one thing.

I've toyed with raising the gain at various statges in all my amps, by biasing hotter and increasing plate voltages, and raising the value of the grid leak resistors, and tube rolling to find the most powerful V1 or V2 preamp tubes and had some good results overall, but I've backed off on changes once I saw what other non-gain related "Changes" could acheive.

Through all that I've come to one overriding conclusion (for me that is), I would rather spend time to change the filtering of the signal first, by trying different size coupling caps, cathode bypass caps, and ensure the filter caps are working properly or new, and adjust the pentode screen voltages high enough, while using screen resistors. Gain at different stages is important to strike a type of harmonic tonal balance, but perhaps I overrated it's importance early on.

I've had the best tonal changes by concentrating on the above filtering and amp health factors first, and not just on boosting the amp's internal gain, and then finally feeding the amp a high frequency and tailored signal up front, furnished by an EQ'd and clean boost pedal. Once you nock back any offending frequencies in the guitar input, and in the amp circuit itself, and deal with any parasitic losses from bad wiring layout, hum, resistors, caps, and bad tubes, most amps will sound great boosted, even without coming to the perfect balance between preamp and power amp gains.

Speakers and cabinets are very important in the quest for a great tone, once you have your amps bigger issues in any amp sorted out, but it pays to work on both things at the same time, that is try different speakers WHILE you are making other tonal mods. A very Complicated process for sure, but you gain insight because our final tone and response is all going to be a blend of the parts, and just trying to overcompensate for defeciencies in a cabinet or speakers is a pyric victory.

Just my limited observations, so take what I said with a grain of salt, as always !

[g1]

Sorry, I stand corrected on R40.
As far as the parallel triode, I did not realize this was the original way the circuit was, and thought it an odd way to do it rather than the standard paralleled triode like used in standard Blackface fender. So easiest to just stick with the stock configuration.

Agree with Helmholtz that cap bypass on R42 will help, and I'll ask the others if something can be done in the R26 mix area to increase recovery signal.

[q9522678]

Enzo,

Will give your suggestions a shot and report back. Will seek to simplify the front end and move the input and depth connections and let you know how it goes.

[q9522678]

Hi

The grids (2 and 7) and cathodes (3 and 8) are tied together. That’s how I read the schematic and both are jumpered. However separate caps come from each tube pin per the original design. Can certainly simplify this by placing a single larger value cap in there attached to the jumpered plates) - I was simply minimising the work involved and working with what was already in there. Noting the plates are tied together at the grid is there still likely to be phase issues with rather two parallel capacitors? In any case will experiment with this.

Why remove the 560k resistor? This was included on notes that accompanied the schematic - suggested jumpering this resistor (unsure why this wasn’t amended on the schematic). Apologies, should have mentioned that.

I also found this blurb where it appears an amp tech had successfully installed the same tank using what appears to be a similar circuit but driving as a cathode follower (NB I tried this also with same results).

"Silvertone Spring Reverb tank mod:
Use a short 3-spring reverb with 800 ohms input: 8EB2C1B.

First build a tank driver:
Re-wire the reverb driver tube as a follower: remove connections to Pin 2 and wire Pin 2 to Pin 7.
Connect one end of a 1.5uf 250V cap to the cathodes. the other end goes to the reverb tank input.

Now fix the reverb recovery circuit: Jumper the 560K resistor and remove the resistor (and sometimes capacitor) that connect to Pin 2 of the reverb recovery tube. Reason is, these parts make a voltage divider that kills the signal. Install a 220K resistor from Pin 2 to ground as was done in AB763."

Again I have tried driving from both the grids or cathodes with similar results. The reverb is there but very quiet.

Will experiment with 560k resistor back in there as can’t hurt..