Magnatone Custom M10

[TelRay]

Hi all and again Happy new year to all!

encouraged by the fact that I know you will have my back with all needed support I took on a new "project amp".

The poor thing seems to have suffered a lot, I'll share with you what I know after a first inspection.



- Model M10
- Serial number 2002
- Pots serial code WEEK 29 to 34 1964
- 8 tube sockets (2x 7189A, 2x 12AU7, 2x 12DW7, 1x12AX7 and 1x??? cannot read this last one but assuming it is another 12AX7)
- All tubes seem to be the original ones (Mullard made for Estey)
- SPKR 8" OXFORD 80L5N-1 16 Ohm 14oz AlNiCo// 465-435 [WEEK 35 1964] // 25-1016
- SPKR 3" McGREGOR Tweeter 25 1015 // 789434 9 [WEEK 34 1964]- seems to be blown, interested into getting a replacement and adding the option of an ON / OFF SWITCH or an ATTENUATION pot (I never had a Guitar amp with a tweeter before)
- The cabinet is divided (compression chamber)
- The REVERB TANK is missing (would like to know the specifications to get a replacement, but not a priority at the moment)
- The POWER TRANSFORMER has been replaced in the past (by a Hammond 369BX)

As before, my intention is to restore it to its former glory.

I have not dared to plug it in and will probably not do until I have completed a few steps which will be:

- Confirming I have the right SCHEMATICS
- Re-cap, new diodes and resistors in the POWER SUPPLY section



I'm starting to put on a Bill of Materials:

RESISTORS:
- 56 KOhm 1W
- 4.7 KOhm 1W
- 3.9 KOhm 2W

CAPACITORS:
- 1 uF 450V
- 100 uF 50V
- .047 uF 600VDC (this is the death capacitor, right? I do not think I need it as the AC Connection is a 3 prong properly grounded to the power supply chassis)
- the 30,30, 40 uF 450V is a can Capacitor I might need to split in 3

DIODES:
- I see 2 different types (INI69X2, CODI-575?), however selecting diodes is not something I am still skilled at.

To be concrete, this is what I need help with

- Tweeter replacement (or fixing?)
- Reverb tank replacement
- Diodes for the power section (a link to MOUSER would be much appreciated)

thank you all in advance for your always welcome advice!

[52 Bill]

The diodes listed in the schematic are too old and underrated for modern specs, just go with 1N4007s as replacements for all of them.

[TelRay]

..go with 1N4007s as replacements for all of them.

loud and clear, thx!

[g1]

Did you test the tweet at it's terminals? I'd think it would have a series capacitor as a crossover, so that cap could throw you off depending how you test.

[TelRay]

Did you test the tweet at it's terminals? I'd think it would have a series capacitor as a crossover, so that cap could throw you off depending how you test.

yes, that was my hope... that the capacitor was KO
but I disconnected the tweeter from the capacitor and the 8" speaker and tested directly

[g1]

That's too bad. May be a tricky one to find a replacement for. You may want to hold off until you are finished with everything and think the amp is lacking in high frequencies. It may be fine without it.

[The Dude]

Parts Express has a large selection of tweeters. I'd think you could find something similar. Also, lots of the old console stereos used similar style tweeters. If you know of any junked out laying around......

[TelRay]

TWEETER:
thank you guys. As G1 says, I'll probably never use the tweeter, never had any on my amps. But... maybe I find something new and I love it. In any case, the good people of Magna thought the amp needed one so... I want to "restore it to its former glory". However, taking into account that it might be to damn bright (distortion through a tweeter might not be the best idea) I am thinking on a TWEETER ON/OFF SWITCH or an ATTENUATOR (like the ones seen on many bass cabinets with tweeters)
@Dude, yepp... I think any junkyard with old stereo consoles might be a good place to look for a replacement (this is no high end ribbon tweeter)
All I know is that I have an 8" Oxford Speaker that is 16 Ohm and the amp is capable of pushing 38 Watt. The tweeter seems to be 3" and 8 Ohm according to this site. Does that make sense? putting a 16 Ohm and 8 Ohm speakers together?
how do I make sure that whatever tweeter I find it's suitable for the job?

[Rhodesplyr]

In terms of output power, the specs you are quoting are misleading. There is no way you can get 38 Watts from a pair of 7189A run ultralinear (or pentode). You might be able to get a brief transient peak of 38 Watts, but in terms of continuous power, it's probably around 19 Watts.

A guitar amp is not HiFi, so if Magnatone chose an 8 Ohm tweeter, it won't hurt anything. It may have been all that was available.

It looks like there are a bunch of 3" paper cone tweeters on eBay. You could choose something that looks similar. There is a slim chance that a vintage speaker reconer might be able to recone the original, but only if they could get the right parts.

From what I've heard, most people using Magnatone amps are not pushing a lot of distortion.

You can still get multi-section capacitors from CE Distribution or Hayseed Hamfest if there isn't a lot of room in the power supply. I'd use a film cap these days for the 1uF 450V.

[Helmholtz]

All I know is that I have an 8" Oxford Speaker that is 16 Ohm and the amp is capable of pushing 38 Watt. The tweeter seems to be 3" and 8 Ohm according to this site. Does that make sense? putting a 16 Ohm and 8 Ohm speakers together?

The 4µ cap in series with the tweeter constitutes a primitive crossover network with a crossover frequency of around 5kHz. The impedance of the cap adds to the tweeter impedance. At 1kHz the total impedance of the tweeter branch is around 40 Ohm. While the cap's impedance falls with increasing frequency, the speaker impedances increase. So I would expect the total load impedance to stay above 8 Ohm at all frequencies.

[TelRay]

In terms of output power, the specs you are quoting are misleading. There is no way you can get 38 Watts from a pair of 7189A run ultralinear (or pentode). You might be able to get a brief transient peak of 38 Watts, but in terms of continuous power, it's probably around 19 Watts.

What you are saying makes sense, but the 38W is what I found on the internet (the link was in the original post). I haven't even turned this thing on to be able to judge

if Magnatone chose an 8 Ohm tweeter, it won't hurt anything.

We can doubt the tweeter is 8 Ohm in the same way we do with the output power, the information is coming from the same internet research and there's no impedance value stamped on the tweeter itself.

You can still get multi-section capacitors from CE Distribution or Hayseed Hamfest if there isn't a lot of room in the power supply.

Yepp, that's what I had in mind. But did not know about the Hayssed, thx

The 4µ cap in series with the tweeter constitutes a primitive crossover network with a crossover frequency of around 5kHz. The impedance of the cap adds to the tweeter impedance. At 1kHz the total impedance of the tweeter branch is around 40 Ohm. While the cap's impedance falls with increasing frequency, the speaker impedances increase. So I would expect the total load impedance to stay above 8 Ohm at all frequencies.

richtig! loud and clear... any lousy 3" 8 Ohm tweeter able to handle 20W will do

[Rhodesplyr]

What you are saying makes sense, but the 38W is what I found on the internet (the link was in the original post). I haven't even turned this thing on to be able to judge

I'm sure you found the 38W figure on the Magnatone site. I read the same thing there. It probably came from Magnatone's original marketing. The thing is, back in those days, the rules about stating an amp's power output were a lot looser than they became in the 1970s. There was a LOT of false advertising in the 1960s (or at least stretching the truth) especially after Fender started building 85 Watt amps. Suddenly, 19 Watts sounded pretty puny. Tube amps can momentarily put out transients in excess of their continuous power, and amp builders started quoting momentary peak power figures vs. continuous power to make their amps appear more powerful to potential buyers in the marketing literature.

I am certain about the limitations of a pair of 7189s because I've been working on tube amps for ~30 years, and I know the limits of particular tubes. You can also look at the published tube datasheets. The most anyone was ever able to squeeze out of a pair of 7189s is ~24 Watts, and that's running them in pentode mode. I have an H.H. Scott 299 integrated HiFi amp, and it can manage 20 Watts clean with the help of adjustable fixed bias per tube and AC balance in pentode mode. Ultralinear outputs like the Magnatones have lower distortion, but at the price of slightly lower power.

When amp builders wanted more real power from a pair of tubes, they had to go to 6L6GCs, EL34s, 7591s, or 6550s. That's what H.H. Scott did. With the 299C, they switched to 7591s.

7189s themselves were an uprated EL84 with higher voltage ratings in the quest for more power from a miniature 9-pin tube.

[TelRay]

that makes a lot of sense

[TelRay]

quick recap:

[SOLVED]
DIODES: I'll go for 1N4007s for all of them
TWEETER: any 3", 8 Ohm 20W will do

[PENDING]
REVERB TANK: any suggestions, is it possible to get the right replacement simple by taking a look at the schematics?
DEATH CAPACITOR: can I remove it (.047 uF 600VDC) as the AC Connection is a 3 prong properly grounded to the power supply chassis?

thx!

[Rhodesplyr]

In terms of the reverb tank, it would be great to find someone with an M10 and original reverb tank to see exactly what was in there.

These were the very early years of reverb tanks. They were invented by the Hammond Organ Co., first appearing in Hammond Organs in ~1960, and you'll find early ones with Hammond on them. They took off in popularity, and Hammond spun manufacturing off to Gibbs to meet demand.

I've been told that there were two common types found at that time, the "C" tank and the "F" tank. I'm told that the "F" tank was for Fender and that it had an 8 Ohm input impedance.

(For years, Hammond had been making reverb systems with 8 Ohm drivers because the early oil-tube reverbs used what was essentially an 8 Ohm speaker without a cone to drive the springs. Hammond used a transformer to match a 6SN7 tube to the 8 Ohm driver, so there was a 20 year precedent for 8 Ohm spring reverb input impedance.)

However, the M10 uses a 12AU7 to drive the tank without a transformer. The input impedance of most reverb tanks is a hard load to drive from the plate of a tube because it's a relatively low impedance for most tubes to drive. That's why both halves of the 12AU7 are paralleled in the M10. I'm only going from what's been told to me, but I've been told that the "C" tank of that era is for a tank driven through a capacitor from a tube, hence the C.

Note that the F and C I'm talking about do not correspond to what you'll find on impedance charts for later tanks. It's very confusing.

The M15A I'm working on does have a tank with F on it, and its input impedance is 8 Ohm; however, it's driven from the speaker output, not from a tube. You probably want the C type, which is associated with Ampeg. If you search Ampeg C reverb tank, you can find some choices.

If anyone finds anything to correct in this account of what F and C mean in terms of early reverb tanks, please do so.

Remove the death cap.