Does anyone know what amp is it?

[chikov]

As I am learning more and more about amps and amp design, I started going to the local guitar store here in Houston, TX called Fuller's Vintage Guitars. I started asking them if they have any tube amps that no one wants to work on, and if they want to let me try to fix it. Well, it worked. I fixed already 1942 Gibson ES 125 (I Think) and 1962 Western Auto Trutone ( see photos below).

But, the one I got today does not have any logo on it or any other ID. Here are the photos.... Does anyone know what kind of beast is it?

[Stevem]

No I don't know , but I would have to guess that since it has no controls from what I can tell by your photos that there is a really good chance it was made during the electric Hawaii guitar phase in the 40s!
The power Transformer at least should be stamped with a 6 diget number code that will let you pin down what year it was made and the speaker and OT may have a date code too!

[chikov]

This amp has a "dynamic" ( fieldcoil?) speaker ( i think that is the proper name for it). It basically has a coil on the outside... it sounds harsh and high pitch... can I replace it with a regular speaker or they do not interchange?...

[Stevem]

You can replace the field coil with a resistor of equal resistance to the coil, and a 10 watter will be more than enough, but you will then have added hum since you now have no choke action to reduce the hum.

Upping the uf value in the power supply filtering to compensate can be done, but only up to the point that the rectifier is rated for in regards to current.

In short it may be better to just replace the field coil with a choke with about 1300 to 1600 ohms and rated for like 70 ma should do it, then you can use any speaker that matches the impeadance of what's in there now.

[Smokebreak]

Here ya go :
http://www.rickresource.com/register/vi ... p?id=19156
http://www.rickresource.com/register/vi ... p?id=35274

Looks like a Rickenbacker from the 30s
Generally speaking you can't just replace a FC speaker with a permanent magnet one, as the FC usually serves as some sort of choke. You can replace it with a large power resistor, and put in a PM speaker. I did that once with an old Epiphone. You can measure the resistance of the FC, and also how much voltage its dropping, and figure out how big your resistor needs to be , I'll guess something in the range of 10W 250ohm.

edit : what Steve said !! ^^^^

[chikov]

Thank you, guys, for this great help. Yes, it is Rickenbacker Model 59 from 1938-1939. Wow. I tested all the caps and found all of them out of specs ( even the "newer" metal can one). Surprisingly, I did not find the schematics of this amp in the internet, so in order to "pay back" to the amp community out there, I drew the schematic by hand. I am attaching the file below, if I made any mistakes, please let me know, but i am certain that it what is in this amp. I wonder if the amp sounds so bad not because of the speaker, but because of all the bad caps...

[rp]

Never occurred to me to walk into a music shop and ask them if they have any has-beens and orphans that need help. Clever idea, great way to get to work on paleolithic amps. Hell, I'd even pay for the parts, there are so few, as long as it's not too much of a desiccated lizard, bug husks and mouse turd chore - hanta virus, you know

Now you need a frying pan, no, not an electric skillet, one of these:

[chikov]

I think these old amps tell the story of the tube amp evolution and help me to learn and understand the physics of that wonderful world.
That brings me to a good report - after replacing all the capacitors I got this amp to work. The only complaint is that it is soooo quiet. I feel hat something is not right. On pin 8 of the 5Z4 i get about 362V, past choke 343V, past Field coil ( on the screens) 270V (quite a drop). On the plates I have about 262V. I have a feeling that most of the electrones are going to 6V6 screen and not the plate. Does it make sense? Is it a design flaw?
PS on the scematics that I drew, i forgot to add 640K grid resistor ( on the imput jack to the ground.

[rp]

Maybe it needs more troubleshooting but just looking at the schematic it was born with very little gain. If you aren't locked into a detailed restoration try a bypass cap across the pre and/or power cathode resistors. Having experimented with old circuits it's best to modernize the input with the typical 1meg load and a 10k-33k stopper. The stopper knocks out a minuscule bit of sparkle but does add hiss, this is so low gain I don't think hiss is a worry.

[rp]

Forgot, one advantage of little gain is you don't need a volume pot

[pdf64]

So there's a choke AND a field coil in the HT supply?
What is the resistance of each? A lot of Vdc seems to be getting lost across them.
The absence of cathode bypass caps shouldn't affect the gain much, as both stages are balanced and have shared cathode resistors, so the ac there should get cancelled out, so eliminating the need for bypass caps.
If operated in class A, it shouldn't reduce the (clean) power output either.

Are you sure that all tubes are good and reasonably matched?

The screen grids being a few V higher than the plates is to be expected with the design.
Theoretically it isn't ideal but many vintage amps are similar.

[chikov]

Guys, thank you for your help. I did check my tubes on my tube tester they all look good. The resistance of my choke is 227 ohms and field coil is 877 ohms. Should I replace a field coil with a resistor and try to use a different 8ohm speaker?

[martin manning]

The resistance of my choke is 227 ohms and field coil is 877 ohms. Should I replace a field coil with a resistor and try to use a different 8ohm speaker?

Should work fine. This amp seems to be what we would now consider just a power stage with no preamp. I'd think running a boost in front would be a good idea.

[rp]

This amp seems to be what we would now consider just a power stage with no preamp. I'd think running a boost in front would be a good idea.

[Phil_S]

Egad! I wanna unsee that, but I can't!