Masco Reel to Reel Conversion

[jeff]

Time to get to work.
This is a Masco Reel to Reel tape recorder someone gave me a long time ago. Bout time I get it running.
It's kinda neat how it works. In 'playback' it uses a 6V6 as the power tube. In record mode it connects the O.T. to ½ of a 6SN7, and uses the 6V6 for the erase head. I figure, I'll keep the 6V6 power tube and, instead of driving the speaker with the ½6SN7, add a Xfrm to drive a reverb tank.

Schematic found here https://www.radiomuseum.org/r/mark_simps_d37.html#images
More pics/schematics to come.

[jeff]

Here’s the schematic in “Record” position
Notice the OT connects to the 6SN7

[jeff]

Here’s the schematic in “Playback” position
Now the 6V6 is connected to the OT

[jeff]

Here’s my idea to use a combination of the two. Using the 6V6 and stock OT as power section, using the 6SN7 and extra OT for reverb

[jeff]

Here’s a better schematic.
Chassis gutted time to get building. It’s basically “Playback” position, but using the unused 6SN7&6SL7 halves for reverb.

[jeff]

Is it OK to move the volume from after the 2nd 12AX7½ to after the 1st? There is neg. feedback between 1st and 2nd stage, will moving the pot interfere with that?
The original schem has the volume after the two 12AX7 stages, but I don't know if that's good for tape playback/bad for guitar.
Should I just omit N.F.B.?
Don't see that on many geetar amps


Thanks
Jeff

[kagliostro]

The use of a gain pot give you the chance to reduce the drive coming out from a stage and going to the following stage

This can alter heavily the situation and avoid saturation

You can be looking for this effect or not

So think to what will happen moving the pot to a further stage and make your considerations

Looking to the various examples of preamps on Blencowe's preamp books, from the lower gain to the higher gain versions you can have an idea about the difference on using a gain pot in various positions

I think Merlin can give you more detailed explanations on the matter

---

Talking about the possible use of the reverb tank transformer driver as a speaker output consider that it is possible (if your choice is for a low impedance input tank) but in your plan you have two further gain stages (3 considering the tank recovery stage), so what you'll be obtaining will be a different version of the sound, I don't know if you will be interested on that or if you will be interested to move the whole reverb circuit further on your circuit as to be the last ring of the amplification gain chain that precedes the Power tube

If the recovery tube of the tank feeds directly the power tube, may be in this way the tone you have at the exit of the reverb driver transformer will be more similar to the tone you have at the exit of the power tube transformer

All is your choice, think to what you want


EDIT: I forgot to say
Consider also to use both the 6SN7 triodes as reverb driver

My 2c, look to existing circuits that are of your interest and plagiarize it

Franco

[jeff]

OK, thanks
Does anyone happen to know how the strength of the tape playback head compares to the strength of a guitar's pickup?


In other words, if you had a tape recorder and replaced the playback head with a jack and plugged a guitar in, would the signal be about the same or larger as playing the tape?


Because, basically, I'm planning to hard wire the amp in 'Playback' mode(all switches B in original schematic) but replace the playback head with the guitar input jack.
In 'Playback' mode ½6SL7 and ½6SN7 are unused, which is why I'm trying to repurpose them for reverb.

[kagliostro]

Not to clip your wings but I doubt in a positive result

Of course you can give a try but I'll not expect much from that

You have some good tubes for audio use, plagiarize proven circuits that uses it in guitar amps

Franco

[shooter]

with 4 gain stages, you should have more than enough to make the 6V6 cry uncle.


I'm not a knob fan on SE amps, i'd leave the gain pot un-wired for future tweaking if required.
good idea to have an AU, AT on hand for swapping into V1 for testing.


NFB is easy to eliminate/add  so leave that wire off for future tweaking if required
There, I saved you 4 wires n 8 solder connections when it's time to trouble-shoot problems 

[jeff]

NFB is easy to eliminate/add  so leave that wire off for future tweaking if required
There, I saved you 4 wires n 8 solder connections when it's time to trouble-shoot problems 

Yep, think I will leave NFB off,
Thanks

[jeff]

I'm not a knob fan on SE amps, i'd leave the gain pot un-wired for future tweaking if required.
good idea to have an AU, AT on hand for swapping into V1 for testing.

Wait...
Took me a sec, are you suggesting wiring a resistor as if it were the volume full up, then adjust volume by swapping tubes?
Just full blast, use volume/tone on guitar?

[shooter]

use volume/tone on guitar

yes
and the pedal board probably has gain-ish things to play with.


a simple SE should be just that, simple, I've found they are really good simple, when paired with the right speaker(s), a good guitarist, a fist full of pedals to choose from, walla, the chick's will dig it.

[jeff]

I'm really sweating using the 6X5. The more I read, the less I want to use it. Notorious for shorting out and blowing the PT. Chance it? I could use SS diodes... but then my voltage will go up. I could add a 6V accessory Xfmr.


Should I just omit the 6X5, use SS diodes, and add a big honking resistor to lower the B+ to what it would be with the 6X5?
Any idea the value of the resistor to simulate the drop in B+ the 6X5 would give?


What would you do? Use it and chance blowin PT if it shorts, or omit it and add a resistor and two SS diodes, or add a 6V supply?
 
This is really stalling me out.
Thanks for the advice.
Jeff

[AlNewman]

Probably the 6x5 is fine, but if you're worried about it, you could just run diodes direct across the socket, and see what the voltages are.  From what I've seen on these old amps, you'll be lucky to break much more than 300v, but maybe your amp is different.

[kagliostro]

May be the use of a pair of diodes in series (not across the socket) with the tube diodes will prevent problems to the circuit if the tube rectifier shorts but maintain the resistance of the tube, this way B+ level will not increase


Franco