Need help with 1966 Kay model 720

[smackoj]

Hola amigos; I have been away for a long time. I found this barn queen for a few pesos at the swap meet. A 1966 Kay model 720. I am trying to bring her back to life but have a couple unknowns needing your advice. I tested the voltages running through a current limiter using only the 5U4 rectifier tube installed. I read 450 vdc at the pins of the can capacitor. Without the other tubes installed, I got the same voltage at the power tubes and the same at the power pins on the 7199 tube socket and the 5879 socket. I don't have my tube tester at this location so I don't know if the tubes are good other than the rectifier. The 6.3 heater voltage is correct. Could you look at this schemo and tell me if this amp makes sense on paper? I noticed it does not have a capacitor on the 6L6 power tubes to help bias them. Gracias amigos.


 

[sluckey]

Could you look at this schemo and tell me if this amp makes sense on paper?

Looks fine to me.

[smackoj]

thanks SL

[bmccowan]

Nice find. A very simple amp - kind of like the Supro Thunderbolt but with a pentode in V1. There is a lot of room in that chassis to make it guitar friendly if that is your intention. Schematic looks like an entry in a fewest parts in a 2x6L6 amp competition.

[PRR]

...I tested the voltages ....only the 5U4 rectifier tube installed. I read 450 vdc at the pins of the can capacitor. Without the other tubes installed, I got the same voltage at the power tubes and the same at the power pins on the 7199 tube socket and the 5879 socket. ....

Of course. With the tubes out, there is NO load on the B+ chain. 450V all down the line.

The power stage cathode cap does not "bias" the amp. It may smooth-out bias variation with signal. In a hot push-pull amp it can often be omitted: the two sides cancel-out and there is almost no variation or bass-cut.

[smackoj]

Yes it follows the KISS logic    Why get too fancy when there are 10 pedals available for whatever sound you desire.

Thanks PRR always like hearing your directions


here is a pic of the Oxford 15" that came with

[tubeswell]

Couple of Kay schematics scanned from my Jack Darr book (Sorry the pictures curled up on the book's binding side). Different model numbers but at first (and second) glances, they looked like the same amp FWIW.

[PRR]

> looked like the same amp

In the power rail, I see a 47 vs a 50. Which is of course insignificant. I didn't follow all the way down the rails. The audio part seems 99.9% identical. Like Camaro/Firebird, twins under the skin?

[smackoj]

Thanks for the added info amigos. Is it true that during the late 50s and 60s, amp companies would use the same amp and designate it a guitar amp, and in other formats or trim also a bass amp? I know that guitar players would mortgage the farm if they found an original Leo Bassman for sale.


Update on the Kay 720. Brought it up to run status with all the tubes in. Voltage on the power tubes around 330 and the preamp tubes around 172. It does not pass a signal to the test speaker though, so I am suspicious of the volume and tone controls. They used 3 long shielded signal wires from the control panel down to the amp. These signal wires are 2-3 ft long and grounded on the pots and the preamp side.

[bmccowan]

Is it true that during the late 50s and 60s, amp companies would use the same amp and designate it a guitar amp, and in other formats or trim also a bass amp?

I was around in those days, and my answer is yes and no. It was hard to get a tube amp to handle bass guitar, so amps designed for bass were relatively simple, but robust with usually 1-15" speaker or famously 4-10" speakers. Many of these amps sound good or great with guitar. Some need simple tweaks to sound great with guitar. Valco had some amps that had bass, guitar, and accordion inputs. Nutshell - a lot of guitar and bass amp circuits from that era were damn similar.
BTW, I suggest you follow a known logical and safe test procedure to determine what this amp needs. You may have one. If not there is a good procedure here on Doug's site.

[smackoj]

thanks bmc; I am waiting on a couple new preamp tubes to arrive. without my tube tester I want to try those. Wiring looks good, voltages are all good and no current issues thru the light bulb limiter.