Silvertone 1484 Restoration - almost there, but need assistance!

[Resamonk]

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.

Hello, first time posting. Been searching for a way to replace the reverb in my Silvertone 1484 and ran across this thread! Been doing a few mods and learning as I go for quite a few years now but still amateur at best. Wondering if there was a chance that any info on the reverb conversion schematic was still around?

Anyway, much appreciation for the shared knowledge on this forum. Thanks for reading.

[Enzo]

Some schematics are harder to find than others. But in following the thread I have found it esiest yo just google SIlvertone 1484 and so many copies of the schematic pop up, I can choose almost at random.

[soundmasterg]

Greg

Stand down in looking for the schematic. I did some googling and found the schematic for voltage doubled bias for the 1484 from Gerard Weber’s book. I have installed this circuit and it works great. Bias now sitting around 34ma and seems to be happy there. Thanks very much for your input and advice.

Time to put this old girl together and enjoying playing through her.

Thanks again hi everyone for your advice and input.Another amp lived to fight another day!

I'm glad you were able to get it all sorted out! I just saw your messages today, so I'm happy I don't have to go look for the schematic.

I've got one of these that is completely stock and reverb and trem surprisingly work perfectly, but the rest of the amp has issues and needs new resistors and caps in some areas. One of these days I'll get around to fixing it.

Enjoy your amp!

Greg

[q9522678]

Greg

Stand down in looking for the schematic. I did some googling and found the schematic for voltage doubled bias for the 1484 from Gerard Weber’s book. I have installed this circuit and it works great. Bias now sitting around 34ma and seems to be happy there. Thanks very much for your input and advice.

Time to put this old girl together and enjoying playing through her.

Thanks again hi everyone for your advice and input.Another amp lived to fight another day!

[q9522678]

Hi Greg,

Thanks for the info. I had stumbled across the documented errors in the schematic - so have been working around those. No problems there.

I have seen your posts on other forums regarding the mods done to your brothers amp. I read this with much interest. I also saw your comments about the voltage doubler for the bias supply, but alas, could not find any schematics and would rather not attempt it without a schematic. I think I have a reasonable idea but would want to be sure before giving it a go. If you wouldn’t mind digging out the schematic that would be much appreciated. I am lazy too - hence the minimalistic reverb upgrade! I’m happy to give the adjustable bias another go using the voltage doubler.

And yeah - had read these are rated 20-25w despite the 60w marketing!

[soundmasterg]

As Enzo said, these amps are non-adjustable fixed bias. There is an error on the commonly available 1484 original schematic. Actually two errors. The first is that they show the grid resistors of the power tubes grounded where the negative bias voltage is coming in. Those are not really grounded there. Second error is on the trem tube...they have pins 6 and 8 reversed. Move the numbers and everything is correct.

The bias supply needs to be voltage doubled and then it will work in the correct range. Add an adjustment pot in there and it works fine. I forget the values I used but it wasn't hard to do. I posted a whole bunch of info on the mods I did to my brother's 1484 over at the Hoffman amps forum maybe 10 years ago. These amps are only about 25 watts RMS btw.

Greg

P.S. If you really want to know what I did for the voltage doubler circuit to get adjustable bias, let me know and I can go look at my schematic in the garage. Too lazy right now to go find it.

[q9522678]

Hi Enzo

I follow this schematic that I found on a 1484 mods website:

https://www.dropbox.com/s/sp0s2mifp9...%2013.jpg?dl=0

[Enzo]

How is your bias pot wired in? Just making the 100k resistor variable won't do it.

There is no grid leak bias in this amp. The power tubes are using fixed bias. Adjustable bias is still fixed bias.

[q9522678]

Hi g1,

I’ve been using a Eurotubes bias probe and a fluke DMM. One I’ve had for around 10 years and use to bias all my other amps. Based on this I can get one about 29ma out of the bias pot(50k pot with a 50k resistor). But likely going to draw a line under this one. Based on the schematic for the bias mod I should be getting between -18vdc and -36vdc from the bias pot and I’m getting -32vdc. So I’m thinking that’s good enough. All preamp, reverb and tremolo voltages are all good. Per previous discussion the voltage doubler and power section voltages are slightly low but am lead to believe nothing to be concerned about. The amp is sound good where it is, with little to no audible hum, even when dimed. The reverb is working great and sounds pretty good. So all in all the amp is going great and sounds good. So I think this one is pretty much done.

I’d like to thank you all for your very helpful input. This has been an enjoyable restoration and I really appreciate the discussion in this thread. Hopefully my update reverb schematic will come in handy to those who want to update the 1484 reverb with something usable at minimal cost and change in parts.

[g1]

What method are you using to measure bias?
If B+ is dropping to the point of hum, you are making a big change in bias.
For some reason there is a problem with the bias measurement and it is not showing the change.

[q9522678]

The 50k linear pot arrived today. I didn’t have a lot of time to play around today but got it installed and did a little bit of testing and adjusting. Appears noting is ever easy as I found the as I lowered the bias pot value I would only get a very slight increase in bias but a notable drop in supply voltage until I got to a point where there was notable background hum.

I’m doing some brief research could be because these amps are grid leak biased as opposed to fixed bias? Did some quick reading on this and that the amp benefits from a “bias voltage doubler” - no as simple as just replacing the bias resistor with a bias pot it would seem. So now considering just putting the 100k resistor back in and leaving as is.

PS I ended up replacing the old diodes. Thought it good insurance. Didn’t change any of the voltages however.

[q9522678]

Once I receive the pot I’ll install and experiment and report back on where I settled in terms of bias.

In the meantime I’ll recheck the voltage doubler circuit to ensure all connections are where they should be and are solid. May consider replacing old diodes in there with IN4007s but generally don’t like replacing parts for the sake of it. Though I suppose there is the argument these are now 55 year old diodes and putting new ones now is a good opportunity and good insurance. Something to cogitate on while wait for the bias post.

[Helmholtz]

Heater voltage is 6.2v and wall voltage 122v.

Now this is just perfect.

Yes, 28mA cathode current looks a bit cold, meaning around 25mA real plate current. You may verify using the OT primary DCR biasing method (but pull the PI tube before measuring plate voltage to avoid oscillation).

[Enzo]

Me too, forget the 70% internet thing. Most amp makers and many techs bias amps cooler than that, there is no right and wrong. Well... So cold you get massive crossover distortion is wrong one way, and so warm the tube get red hot is wrong the other way. Between those, whatever sounds good IS good. it is just a guitar amp, nothing precision about it.

[q9522678]

You’re singing my tune.

I’d just ordered a 50k linear taper pot just for that purpose.

I’m doing some reading I am lead to believe these amps were generally biased cold from the factory. Which is why the power tubes last in them so long - and many still have the stock power tubes going strong.

I’m not religiously about the 70% I generally use it as a guide. I usually bias largely by ear once in the ballpark to what sounds good to me.

The bias pot will enable this to be done a little easier.