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Thread: Silvertone 1484 Restoration - almost there, but need assistance!

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    Silvertone 1484 Restoration - almost there, but need assistance!

    Hi All

    First post here, hoping you call help me with what Iím hoping are the last stages of a tube amp restoration. Iíve been lucky enough to be given a Silvertone 1484 Twin 12 piggyback head in stock condition which appears to have been stored quite well. The MDF head shell is still intact, the tolex is good and not a lot of rust or any major degradation from a cosmetic perspective.

    I have been working on restoring this amp for the past few weeks. Iíve replaced the two prong power cord with a three prong cord, installed a new fuse holder, installed a speaker out jack, replaced all electrolytics throughout and replaced all out of spec carbon comp resistors with new 1w carbon films. I tested remaining resistors and caps for noise and other issues and they seem fine (not that there are many left!!! ).

    I replaced all tubes with known good ones for now- simply for the purposes of testing, to rule them out as the cause of any other issues. The amp came with all original Silvertone tubes- which will be going back I once the restoration is complete (provided they are functional!).

    Now the amp fires up and plays. It is very quiet , no concerning hum or anything like that. My only concern is it doesnít sound like 25w in terms of volume and only mildly breaks up, even with a hot humbucker guitar, which is odd for a 1484.

    I tested all key voltages and many seem a little low. I live in Australian and this is a US amp. Iím using a step down transformer for wall voltage. Used my DMM to confirm this is pumping out 122v AC.

    In terms of amp voltages I measure the following:

    Heaters - 3.2v (should be 6.3v)
    Pin 3 of 6L6s - 447v (should be 475v)
    Output of voltage doubler (C26) - 449v (should be 480v).
    V1 - Pin 1 - 135v (should be 100v) Pin 6 - 135v (Should be 100v)
    V2 - Pin 1 - 87v ( should be 93v) Pin 6 - 88v (should be 93v)
    V3 - Pin 1 215v ( Should be 225v) Pin 6 - 103v (should be 100v)
    V4 - Pin 1 - 56v (Should be 55v) Pin 6 - 130v (should be 160v)
    V5 - Pin 1 206v (should be 215v) Pin 6 - 201v (Should be 215v)

    Cap can replacement voltages:

    22uf (C25C) - 329v (should be 340v)
    10uf (C25B) - 173v
    8uf (C25A) - 224v

    My main concerns are the low heater wire voltage - at around half where it should be, the low power tube voltages on pin 4, and the high voltages on V1. Iím guessing this is contributing to the lower volume.

    The amp plays fine, just quieter and less raucous than others I have played. Could this be a Power Transformer issue?

    The stock reverb was dead (not a big deal...) . One of the piezo pieces had snapped off. Iíve started working on installing a new Accutronics spring reverb ( 8EB2C1B - 800ohm input impedance) using the existing reverb tubes for drive and recovery. Is working very faintly. Needs some tweaking but have shelved the reverb install until I sort the voltages in case this is affecting the strength of the signal being pumped into the tank,

    Open to any suggestions? Ideally would like to get the voltages closer to where they should be - particularly the heaters.

    Many thanks

    Paul

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    Lifetime Member Enzo's Avatar
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    Hi

    Your heaters have 6.3v if they light up and it amplifies. If you really had only 3v, it wouldn't work. You are taking readings to ground, which is either side of the center tapped winding. Only thing that matters is that you have 6v ACROSS the tube heater, NOT to ground.

    The tube DC voltages do not bother me at all, really. This is just a guitar amp, nothing precision about it.

    As to power supply, the 340v is on C25A, not C25C. And I think you have the other two out of order. I see no way for C25C to be higher than C25B. In any case, I see no problems. The fact they are out of order concerns me that you may have wired them in the wrong positions. The A, B, C sections are in decreasing capacitance and decreasing circuit voltage, in that order.

    Could this be a Power Transformer issue?
    No. Power transformer failure is THE single least likely failure in any amp.

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    Don't forget the joker g1's Avatar
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    Agree with Enzo, most of those voltages are in the ballpark.
    When you say other 1484's have more gain, do you mean ones you have tried with this guitar?
    As far as the main B+ voltage, if the power tubes are biased too hot, that could pull down the supply voltage. Do you have a way to check the idle current of the power tubes?
    And can you post voltages for pins 3 & 8 of the preamp tubes you listed?

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    Quote Originally Posted by Enzo View Post
    Hi

    Your heaters have 6.3v if they light up and it amplifies. If you really had only 3v, it wouldn't work. You are taking readings to ground, which is either side of the center tapped winding. Only thing that matters is that you have 6v ACROSS the tube heater, NOT to ground.

    The tube DC voltages do not bother me at all, really. This is just a guitar amp, nothing precision about it.

    As to power supply, the 340v is on C25A, not C25C. And I think you have the other two out of order. I see no way for C25C to be higher than C25B. In any case, I see no problems. The fact they are out of order concerns me that you may have wired them in the wrong positions. The A, B, C sections are in decreasing capacitance and decreasing circuit voltage, in that order.



    No. Power transformer failure is THE single least likely failure in any amp.

    Hi Enzo

    Thanks for the prompt and detailed reply.

    Reference the heater voltages, thatís a relief. You are correct - I was measuring to ground and not across the pins. Slipped my mind to measure that way, first amp Iíve working on in 4 years (had previously given up fixing amps as preferred spending my limited time playing rather than fixing).

    With the cap can replacements caps, yes, the 22UF is ďAĒ but you may be right in that the smaller two are wired back to front as I recall the 8uf (C) measuring higher than B. Will need to check will I get home. I thought Iíd traced the wires and checked the circuit previously but may have missed it.

    Good to know that nothing stands out as significantly amiss with voltages.

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    Hi g1

    I played a couple of 1484s a few years ago- different room, different guitar. Appreciate there are many variables both internal to the same tube amps, let alone considering the guitar and other environmental factors.

    I remember those amps have quite a bit of grit and dirt when dimed. This one sounds almost glassy - with more headroom. It doesnít sound bad. Just different to previous ones Iíve played. If thatís the character of this amp - all good. I can certainly live with it!

    I send through pin 3/8 voltages when I get home.

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    Quote Originally Posted by q9522678 View Post
    Hi g1

    I played a couple of 1484s a few years ago- different room, different guitar. Appreciate there are many variables both internal to the same tube amps, let alone considering the guitar and other environmental factors.

    I remember those amps have quite a bit of grit and dirt when dimed. This one sounds almost glassy - with more headroom. It doesnít sound bad. Just different to previous ones Iíve played. If thatís the character of this amp - all good. I can certainly live with it!

    I send through pin 3/8 voltages when I get home.
    Pin 3 and 8 voltages respectively:

    V1 - 1.16v and 1.45v
    V2 - 0.67v and 0.56v
    V3 - 117v and 3.32v

    V4 - 0.69v and 0.9v
    V5 - 7.06v (pin 3 and 8 jumpered)

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    I just checked the power supply caps.

    The circuit looks wired ok - however I did note that one the 68k resistors that bridge the power supply caps was in the wrong place - 20uf and 8uf were wired together instead of 20uf and 10uf which would explain the whacky voltages here and likely V1. Havenít had a chance to rewire and retake voltages across the as yet, but will post back when I do, but suspect this will bring most back into the advertised range in the schematic.

    Next challenge is the new reverb tank. Iím using a proposed schematic I found online to convert V4 and V5 to drive a reverb high impedance reverb tank. Whilst it seems to be working - is quite faint. If I turn the channel 2 volume down and turn the reverb up, itís certainly there. And with channel 2 mixed in you can here the reverb but certainly not loud or washy. Seeming like the front end being driven enough. Iím by no means a reverb expert (this is my first attempt) - but what are the standard ways for boosting the input?

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    This is the reverb circuit I am currently using for inspiration:

    https://www.dropbox.com/s/nufi88kd1k...%2049.jpg?dl=0

    Note I have removed and jumpered R40 (560k).

    The tank I am attempting to mate with it is an Accutronics 8EB2C1B. Whilst there appears to be quite a bit of discussion online about replacing the stock piezo reverb with a non piezo tank but no real detail regarding how this is done. Many appear to go with Ampeg VT style circuits.

    I think the circuit I am using was drafted but not necessarily tested. Open to any suggestions / observations.

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    Don't forget the joker g1's Avatar
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    What do you get with R40 (560K) in circuit? I think shorting it out makes the reverb tank output transducer almost short that V1B grid to ground.
    Also, not sure why they didn't just use standard parallel triode to drive the tank (tie plates together instead of running to 2 separate coupling caps). The way it is, any cap differences can lead to phase cancellations. With plates tied together you would need to adjust the plate resistor value.
    But first, just try with the 560K in there, and maybe adjust its value to taste.

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    Lifetime Member Enzo's Avatar
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    Take a step back and look. Removing 560k doesn't load the grid, with the 560k, there is a voltage divider. The signal from the reverb device was piezo crystal, fairly hot. Signal level divides down via 560k over 68k R41. Short 560k, and now full device voltage hits grid. Probably a good idea as I suspect a real pan produces a lot less signal than the old piezo.

    I agree, the whole double V2 thing seems pointless. It is just the original circuit bent to fit...poorly. If you want to try the original circuit, just eliminate V2b, R46 and C19. C18 to the drive end of the pan, and ground the other lead from the pan drive. Putting the extra triode in parallel with the first ought to be a good idea too.

    And do yourself a favor and move the depth control over to where R40 is in the return circuit. Then just wire the drive circuit input up full on, and important: move the source to after the volume control.


    Alternative: consider the original drive was a push pull. R47 provides out of phase signal for lower triode. If you use a reverb pan with isolated input jack, you could wire it up like the old device. No ground connection to drive side. Signal might be too hot for the pan, but is easily reduced with a resistor.

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    I think shorting it out makes the reverb tank output transducer almost short that V1B grid to ground.
    No problem with that. The V1B grid sits on 0 DCV anyway. It will load down the output transducer via R41 (68k), though, causing reverb treble loss with an inductive transducer.

    ...any cap differences can lead to phase cancellations..
    Hardly. While there may be a small signal current phase difference at low frequencies, "phase cancellation" (meaning destructive interference) would require a phase difference of more than 90į which doesn't seem possible with this circuit. Up to 90į phase difference the 2 currents positively add.
    Don't know why they choose this driver circuit. But the 2 triodes are actually wired in parallel for AC signals. Maybe the reverb transformer was not suitable for primary DC current (no air-gap) and/ or connecting the plates would have caused instabilities.

    Bypassing R42 will increase reverb return level.

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    Last edited by Helmholtz; 11-10-2019 at 10:48 PM.
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    Hi

    The grids (2 and 7) and cathodes (3 and 8) are tied together. Thatís 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 wasnít 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 canít hurt..

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    Last edited by q9522678; 11-10-2019 at 10:34 PM.

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    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.

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    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.

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    Member HaroldBrooks's Avatar
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    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 !

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    Last edited by HaroldBrooks; 11-11-2019 at 05:01 AM.

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    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 Iím plugging it to pin 7 of V4 - where the Reverb depth used to be connected. So itís fed in it entirety through the V5 drive circuit.

    Will also try some different values for bypassing R42.. Iím not overly fussy with the reverb tank in terms of sound. Some would say it couldnít be much worse than the piezo reverb - but so long as it sounds ok Iím happy. Otherwise I wouldíve 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 Iíll amend the schematic and post for others to use for others who wish to take the ďsimpleĒ route for replacing the stock reverb (although it couldíve 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.

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    Last edited by q9522678; 11-11-2019 at 06:10 AM.

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    Hi Harold,

    Absolutely agree. Iím 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 wasnít my intent to do this, just happened that way. Now Iíve 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 Iím satisfied the amp is functioning as it should, I donít 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!

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    Lifetime Member Enzo's Avatar
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    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.

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    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 doesnít sound too bad. Iím pleasantly surprised...

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    Lifetime Member Enzo's Avatar
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    Tack the wire right to the volume wiper, see how that works.

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    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...

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    So tried going straight from the volume wiper. Didnít 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 didnít 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 couldnít 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 ď9Ē position on the reverb control the sound goes fully saturated. At around ď9.5Ē 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?

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    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 2.2Ķ/63V with R42=10k.

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    Last edited by Helmholtz; 11-11-2019 at 04:09 PM.
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  24. #24
    Lifetime Member Enzo's Avatar
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    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

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    Quote Originally Posted by Enzo View Post
    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.

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    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

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    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?

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    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?

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    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 0.002Ķ (2nF) cap across the 220k grid resistor (i.e. between grid and ground) of V4b.

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    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!!

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    Alrighty

    So I have attempted to do an updated schematic in terms of how the reverb circuit looks, with some notes, to try and make it easier for anyone who wants to replace the stock reverb in the Silvertone 1484 with a transformerless tube / current driven reverb using minimal parts and minimal changes to the existing circuit. When you take a look, you can see it takes minimal changes. The cost of the new reverb tank is the biggest. Total cost of this mod is around USD$30 if you do it yourself. Other bits are simply RCA connectors, a few caps and resistors.

    So far this is what I have:

    https://www.dropbox.com/s/amj34ls9zb...%2015.jpg?dl=0

    I think I have captured everything thus far. Lines and text in red are changes or additions to the stock circuit. Keen on any input you might have. Wanting to get this right so others can relatively easily follow it.

    I have installed the 100k grid stopper resistor on the audio in - this didn’t make much difference to the reverb control dropping audio at full. Thought it might be some sort of oscillation so installed a shielded cable run for the reverb audio in and this solved the issue.

    I then put a 0.002 if across the 220k resistor and a 2.2uf/63v across the 10k resistor - and voila, the reverb works and sounds great. So mission accomplished!

    Let me know of any suggested changes to the diagram so I can finalise for use by others.

    Before I stitch it up I’ll go through and do a final voltage check - and check the bias on the tubes. Still can’t help but feel it should be louder. I have a 25w Supro that is quite a bit louder.

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    I thought I would take a closer look at the bias for this amp before putting it all back together.

    So noting plate dissipation of 6L6 is 25w and DC plate voltage from pin 3 of power tubes is around 450v:

    25 / 450 = 0.055 x 0.7 = 0.0388

    So should be around 39ma?

    I measure 28ma using a euro tubes bias probe. So seems biased a bit too cold - which I guess could explain quieter temperament of this amp.

    From the schematic appears R60 is the bias resistor?

    Before I start playing around with this to try and get bias a little hotter, does my logic seem sound?

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    Just a small thing to bring up is that the schematic shows 6L6GC as the tubes. What version of 6L6's are in there now. They might be rated at 30 watts and not 25 watts. I might have missed something earlier in thread but just thought to point it out.

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    Actually that’s a good point.

    It presently has Ruby tubes 6L6GC, which I believe are relabelled Shuguang. I’ll have to look those up as I suspect they are closer to 30 watts. Which would put the bias even higher. Although I do intend to put the original Silvertone ones back in as they seem to be working AOK.

    Some current power tube voltages:

    V7
    Pin 3: 451v (schematic says this should be 480v)
    Pin 4: 327v
    Pin 5: -31.8v

    V8
    Pin 3: 453v (schematic says this should be 480v)
    Pin 4: 327v
    Pin 5: 31.7v

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    Strange that plate voltage readings appear low and increasing the bias will only lower plate voltage even lower. At -31v bias supply I would expect higher bias readings from the tubes that you got from the bias probe. I am pretty sure that decreasing r60 sets the bias supply lower, thus bleeding more voltage to ground. That should get the bias hotter. Also the old tubes might bias differently than what you have in there now. You can always measure the bias of the tubes using the current shunt method too, remove the PI tube if you check it that way.

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