Magnatone 421 Tremelo & Vibrato Issue

[plexi50]

I recapped a Magnatone 421 amp today and it sounds very nice and has some good sounding breakup. Problem is the tremelo and Vibrato dont work. I have checked all the resistor and so far they are all within close tolerances. I may have to lift the ends of a few resistors to get the proper reading out of cicuit. I cant find the schematic as of yet

[John]

*deleted huge image.. sorry!*

http://www.bustedgear.com/images/schematics/schematic_Magnatone_421.gif

That's what I found; hope it helps!

[plexi50]

John my magnatone 421 thanks you as do i 

[PRR]

Do you know the ELL80 tube is worth more than the amp?

[plexi50]

Do you know the ELL80 tube is worth more than the amp?

Never seen this amp before. Was this a bad design?

[sluckey]

I think PRR was commenting about how rare that tube is today. About the only place to find them is on eBay. But hey, that pair of varistors are much more valuable than that tube.

About your problem... Since neither trem nor vib work, look for something common such as the oscillator tube, associated components, or switch. Not much to go bad. Should be a quick fix. Measure the plate voltage on the oscillator (V3 pin 6). If it is a steady dc level, the oscillator ain't working. If the voltage is fluctuating, the oscillator is working and you need to look beyond the plate, ie, coupling cap, INT pot, FS jack, or MODULATION SELECTOR switch.

[John]

Do you know the ELL80 tube is worth more than the amp?

Never seen this amp before. Was this a bad design?

From what little I read, some people think the Magnatone was actually a finer amp than the equivalent Fender, but never got the cachet. For whatever that's worth (certainly less than that ELL80) 

[plexi50]

The voltage on pin 6 of V3 is 19.00 VDC and stationary. 170 VDC on the PS node side. Schem says 220 VDC ??

The 270K resistor on pin 6 of V3B was reading 240M. I took it out of circuit and it reads the same. Repaced with 270K

All other resistors in the amp read thier correct values

I am working on the .01 and other caps in this circuit now

[plexi50]

Working. There is an astounding difference between the tremelo and vibrato. Either one is beautiful. The tremelo is harsher in the pulse and the vibrato is smoother and sweeter. Plate pin 6 270K resistor was bad. Quess what it was?

[Willabe]

Sweet.

Hows it sound compared to the Warbler?


     Thanks,    Brad         

[sluckey]

Plate pin 6 270K resistor was bad. Quess what it was?

You said it measured 240M.

If the pic is a clue, I'd guess V2 and V3 were in the wrong holes.

[plexi50]

Plate pin 6 270K resistor was bad. Quess what it was?

You said it measured 240M.

If the pic is a clue, I'd guess V2 and V3 were in the wrong holes.

Yes. A 12AU7 was in V2 and a 12AX7 was in V3. I put the preamp tubes in the wrong sockets.  How can a 270K resistor read 2M removed from the board? The color bands are 270K

[sluckey]

A 12AU7 was in V2 and a 12AX7 was in V3.

That's the correct position.

How can a 270K resistor read 2M removed from the board?

It's carbon comp. Pack the carbon particles tight for small value resistors. Pack'em loose for higher value resistors. When exposed to too much heat, carbon particles expand, becoming less tightly packed, thus increasing in resistance. Heck, carbon is ever expanding, just like the universe. A cc resistor will increase in value even if it's sitting on the shelf.

[plexi50]

Im sorry i have it backwards. The 12AX7 was in V2 and 12AU7 in V3. After changing the 270K i saw the tube chart in the amp and found my error. Yes on the expanding carbon and value range. But i have never seen one drift that far out of spec. It's a first