weird component in Magnatone 260A

[sorin65]

I'm looking to restore an old 50s era Magnatone 260. I found some weird components, never seen before. please help identify them- in the schematic are between anode and cathode for the second triode for each 6CG7 tubes. in the pictures is about the square plastics with colored dots. any clue? thabks!


schematic here:
http://magnatoneamps.com/schematics/...ne_260A_dg.jpg

[dstrat]

Probably the 820MMF
or 820pf

[Bloomfield]

Look up 'domino' capacitors and you'll find a guide to the colour codes.

Here's one: http://www.radioremembered.org/capcode.htm

[Steve A.]

I'm looking to restore an old 50s era Magnatone 260. I found some weird components, never seen before. please help identify them- in the schematic are between anode and cathode for the second triode for each 6CG7 tubes. in the pictures is about the square plastics with colored dots. any clue? thabks!
[ATTACH=CONFIG]47456[/ATTACH]

schematic here:
http://magnatoneamps.com/schematics/...ne_260A_dg.jpg

Wow, that picture is so strange that I thought I better enlarge it....



I've been reading about Magnatone amps for 20+ years but never realized that they were point-to-point and looked like THAT!

Steve A.

[Enzo]

Looks like the insides of any TV, radio, hifi, of the 1950s and 1960s.

When you don't know what a part is, and you already have the schematic, then just look where the part is wired to.

I would agree they are the 820pf from the schematic, plus the color code on the part says 820.

I have to admit I have never seen caps held down with big washers like that before.

[sorin65]

thanks for the answers, but I make a mistake... the components that I don't know what are, are the two brown "tubes" that goes from the 820pF capacitors to the cathode and anode of each second triode of 6CG7. even on schematics looks weird. take a look! also I measured each one, are not resistor, are not capacitor,look as open circuit.

[Tom Phillips]

Those are varistors. Part of the unique Magnatone vibrato circuit. Be very careful with them. They are hard to find and expensive when you can find them.

[52 Bill]

thanks for the answers, but I make a mistake... the components that I don't know what are, are the two brown "tubes" that goes from the 820pF capacitors to the cathode and anode of each second triode of 6CG7. even on schematics looks weird. take a look! also I measured each one, are not resistor, are not capacitor,look as open circuit.[ATTACH=CONFIG]47461[/ATTACH]

Those are the Varistors or voltage controlled resistors that made the Magnatone vibrato work.

I see that Tom has answered already.

[soundmasterg]

Those are the Varistors or voltage controlled resistors that made the Magnatone vibrato work.

I see that Tom has answered already.

Those varistors are VERY hard to find these days. You can find varistors, but ones that have the correct specs are hard to find. Some guys on the Hoffman bbs made it a mission to find other solutions to getting the vibrator circuit functional instead of using the varistors, to varying success. Anyway there is a couple threads there about that. The ones in your amp are one type that was used. There is another type that looks more like a multicolored ceramic cap....I have some of those original varistors myself....enough to build one amp.

Greg

[Mick Bailey]

Out of interest, has anyone here built a Magnatone-style amp with the vibrato circuit, and maybe using a replacement for the original varistors using MOVs and resistors?

[Steve A.]

Out of interest, has anyone here built a Magnatone-style amp with the vibrato circuit, and maybe using a replacement for the original varistors using MOVs and resistors?

With so many rotating speaker pedals out there I wish that there was more attention paid to Magnatone vibrato besides the Boss VB-2 (Behringer UV300) which Lonnie Mack toured with in the 80's.The Keeley Seafoam+ captures it fairly well (Move over, Lonnie!)

[soundmasterg]

Out of interest, has anyone here built a Magnatone-style amp with the vibrato circuit, and maybe using a replacement for the original varistors using MOVs and resistors?

As I said above, a bunch of guys from the Hoffman forum built some clones using other than varistors, or also different varistors. There are several threads on it there.

Greg

[trobbins]

Out of interest, has anyone here built a Magnatone-style amp with the vibrato circuit, and maybe using a replacement for the original varistors using MOVs and resistors?

Yup, I have a clone amp with the original varistors, and have cloned the varistors as well.

Note that it is difficult to check/measure their operation, and they typically never fail, so if there is a problem with your vibrato then don't assume it is the varistors (leave them well alone).

[trobbins]

Article on those varistors:
https://www.dalmura.com.au/static/Ma...o%20design.pdf

[nickb]

Article on those varistors:
https://www.dalmura.com.au/static/Ma...o%20design.pdf

I see you've been busy. Nice work Tim.