Magnatone 210 / 410 build

[woodandglue]

Hi all!
I built a Magnatone 210 /410 with reverb added and it all works, except the vibrato. Ha! There is no noticeable wa-wa-wa except from me.
I measured the oscillator's plate voltage and it is certainly changing. The grid voltage on the oscillator is supposed to be -2.2v and I measure 1v. Do I need to change that, and if so, how? I've checked for continuity throughout the circuit and checked for shorts to ground all over. I’ve rolled different 12au7 and 6N2P (Russian 12ax7 with different heater pins). I've included the schematic as built and included the voltages from both the original schematic and from the amp. The top red voltages are from and the bottom voltages are what is on the original schematic.
This is a repurposed radio so I needed to change some resistor values in the power section to get the voltages closer to the schematic. I also removed the .02 capacitor between the varistors (as suggested by Psionic here ), but have experienced this issue with and without this cap. If there is a thread where the answers already exist, I will gladly delete this one. Your insights are greatly appreciated. I will gladly measure or check just about anything you ask me to!

 

[woodandglue]

This hobby requires more cognition than I have most of the time, but after sending out the white flag yesterday, I discovered where one part of my layout did not match the schematic. I alligatored in a connection and heard the wa-wa-wa coming from the amp, not me!

I would like the oscillations to be slower and faster for the vibrato and increase its depth (of course). Off to dig through more tdpri threads!
Any tips on that or thoughts in general gladly accepted!

If I get it sounding a bit better, I'll be glad to share the differences between the varistors I got from Zack Engineering and the varistor replacements that were discovered and documented by Martin Manning et al.

 

[NTC]

I would be interested in the replacement varistors. I have a Silvertone organ amplifier that is very similar to the 210. I have always wanted to characterize the varistors in order to find a replacement - the characteristics can't be that uncommon.

 

[woodandglue]

I would be interested in the replacement varistors. I have a Silvertone organ amplifier that is very similar to the 210. I have always wanted to characterize the varistors in order to find a replacement - the characteristics can't be that uncommon.

The combination of resistors and MOV is pretty good, but has just a smidge of wump (not thump). I'll have more to report when I get the unwanted distortion under control, hopefully sometime this week. The NOS (I guess) replacement varistors from zack engineering were just under $10 each US shipped and don't have the wump.

 

[woodandglue]

I would be interested in the replacement varistors. I have a Silvertone organ amplifier that is very similar to the 210. I have always wanted to characterize the varistors in order to find a replacement - the characteristics can't be that uncommon.

Also, do you have a schematic for your silvertone amp? I'm curious if and how it differs from this circuit. Does the vibrato speed offer a wide range?

 

[corliss1]

Trem speed can be adjusted by changing the values of the caps in the oscillator circuit. Bigger caps = slower speed. Try some .03 since they already seem to be all .02.

 

[woodandglue]

Trem speed can be adjusted by changing the values of the caps in the oscillator circuit. Bigger caps = slower speed. Try some .03 since they already seem to be all .02.

Thanks! I'll give that a try tomorrow!

 

[NTC]

Silvertone 4707 Organ Amp

I've got my self a Silvertone 4707 from goodwill, with an interesting tremolo, however it has issues! The only sound I can get out of it is a loud hum, that is only affected slightly by the volume pot. The volume only turns it down a bit even on zero, and the speed pot does nothing it seems. It...

www.tdpri.com

This thread has information and a schematic. As built, it does not have a speed control, but the oscillator is the same type in most Fender amps. You can look at those to see how the speed control is rigged. I have never finished with my 4707 - the speed control is just on wires inside the back.

 

[NTC]

You can use a much larger value of pot - Fenders have 2M and 3M pots. That will also slow things down.

 

[woodandglue]

You can use a much larger value of pot - Fenders have 2M and 3M pots. That will also slow things down.

Thanks! I jumpered in a 1 meg pot last night but didn't notice much difference. I bet the larger values would make a more significant change. Now with a new thread to look at I have some more reading and maybe ordering...

Edit: I rejumpered in a 1 meg and it did slow it down, but I want slower, if possible. I put .02 uf in to replace the .01uf in the oscillator but installing 2 or even 1 killed the oscillation.

I'd still like to improve the intensity if possible, but I know that the two varistor vibrato is limited.

 

[woodandglue]

I did some experiments during this build and I can't be sure if the juice was worth the squeeze with all of them, but I can at least rest easy. I've read enough threads that just dead end (including some of my own) that I want to at least show that it can be done.
I replaced the stock oscillator with a modded one from a Swart clone I built that was slow enough for me.
I added a cathode bypass switch with options for stock (none), .1uF and 10uF.
I also threw in a master volume to roar at a whisper.

I would recommend the 4 varistor circuit used in other Magnatones, as the vibrato just isn't as powerful at slow speeds as I would like. I am very open to thoughts on improving this (yes, I already have a red LED on the cathode), but I accept that I can't always make a purse from a pig's ear.