Magnatone MP1 help

[davohilts]

I've replaced the tubes, caps and misc out of tolerance resistors in this amp. When I got it it was weak powered and distorted badly. The tremolo doesn't work even though I can see the LDR flashing.
I suspect a possible issue with the OT or possible miswiring of it.
Main symptom I have, besides the above mentioned issues, is when playing thru the amp with the volume on full the output will slowly go to zero like someone is slowly turning down the volume dial, then slowly go back up to its original level.
Does anyone have a direction to point me??
Thanks
Dave
I'll attach a pic of the OT
http://music-electronics-forum.com/a...&thumb=1&stc=1

[lowell]

Sounds like a bad filament or power supply solder joint or connection with the fading in and out. Tremelo most likely is the 3 feedback caps around the tremelo tube. Also check the power tube bias so we can be sure where we're at here.

not sure about OT. If it were me i'd check dc resistance of 1 terminal of the top 3 terminals to the other terminals. If 2 of them are close within 5-10 ohms then I'd consider the shared terminal to be the center tap of the primary. Try this on the bottom 3 too. Once you figure out the primary you can then figure out the secondaries.

[davohilts]

thanks lowell, I've changed out all the caps, and I mean all in this amp. My concern is the LDR is somehow bad even though it is flashing but who knows. I will check the OT with an ohm meter and reflow solder on all my filament and power supply connections and see what happens.
Dave

[davohilts]

I have checked the tube bias by putting the pos. lead of a multimeter on the ct of the PT and the neg. lead on the plate of the tube (pin 7)
I get 7.7mA on v5 and 9.6mA on v6 using my new tubes
I get 16.2 mA on V5 and 11.4mA on v6 using my old tubes
I checked the plate voltage on all the tubes and get the following:
V1: 182V pins 1&6 (12AX7A)
V2: 363V pins 1&6 (12AU7A)
V3: 130V pins 1&6 (12AX7A)
V4: 244V pin 1 (12AU7A)
V5 & V6: 424V pin 7 420V pin 6 -18V pin 9

Does any one know what the proper bias for these 7189A power tubes, I'm thinking somewhere around 28 mA but don't really know.
I've hooked up a 1M ohm pot across the bias voltage resistor and can adjust the bias to what I need, then I'll measure the resistance and pop in another resistor to set my bias. i just don't know where to set it. 424V on the plate seems like it exceeds the spec for the tube. My head hurts.
Thanks for any help.

[lowell]

"I have checked the tube bias by putting the pos. lead of a multimeter on the ct of the PT and the neg. lead on the plate of the tube (pin 7)"

I'm assuming you meant to write OT, not PT, otherwise your meter fuse would've blown.

So you got the OT wired up then?

It shouldn't be hard to look up 7189 maximum dissipation figures. Take that figure and divide it by the plate voltage, that's your maximum current. P/V=I. I wouldn't worry about the Va, as current is what's important.

Is the amp still cutting in and out?

[davohilts]

hi Lowell,
yes, i've got the OT wired up and I did mean OT not PT.
7189A:
The max. plate dissipation is 13.2 watts
The max. screen dissipation is 2.2 watts
Max Plate Voltage is 440V
Max Screen Voltage is 400V
Max DC Cathode Current 72 mA

So do I take 13.2 Watts and divide by 440V = .03? What does this tell me?
The amp doesn't cut in and out as I found I had installed a 12AU7 in V3 by mistake which should be a 12AX7. When I put in the right tube this cutting in/out went away and my voltages were changed on that tube.
The tremolo doesn't work at all and the amp is still very low gain/volume.
I biased my tubes at 16 which I think is about right.
Dave
The

[lowell]

Hi Dave,
Yes 13.2/440=.03 which equals 30milliamps. In a fixed bias amp you should be fine around 70% of that, if it's cathode biased, which my guess is that it is, you can go 90-100% depending on the sound you want. 27ma seems about right to me as I like warm-biased power tubes. By 16 do you mean 16ma? Take a listen at that setting, you may find you like it or that you want to rebias to something else. Make sure you check the bias for BOTH power tubes. Lets get the power amp functional the best of our knowlegde before troubleshooting the low volume issue.
Have you replaced the 3 coupling caps around the tremelo tube? If it's Fender's style of tremelo then that may fix it. Do you have a schematic for this amp? I'm not sure what kind of tremelo the amp has so a schematic would help. That's good that you figured out the intermittent issue.

[davohilts]

it is fixed bias, what I can do is put in a 1 meg potentiometer in parallel with the existing resistor and adjust the bias to where I want it and then measure the resistance on the potentiometer and solder in a resister of that value, that should do the trick don't you think. That puts me at 21mA or there abouts.
Once I get this done i'm not holding out hope that this is gonna give me the boost in volume, I'm thinking that maybe the OT is hooked up wrong and I'm gonna do some testing to see if I can get the turns ratio worked out and figure out which connection is gonna give me the proper impedance for an 8 ohm load.
thanks for all your help.
Dave

[lowell]

I agree. But yeah your right as far as setting the bias. I thought you already figured out the OT?

[davohilts]

I think I have the OT issue resolved but with 6 options for connection for the secondary I want to read the values and figure which ones are the right ones for an 8 ohm connection.
thanks again for all you help.

[lowell]

Do you need help figuring out the OT or you got it?

[davohilts]

yes, I could use some help, I've got 15 possible combinations for taps on the secondary side and I'm not sure how to figure it out. I've found out I have a Thordarson transformer. It is 24S42 model, a universal transformer rated at 24W. The overall turns ratio is 15.9 to 1 which I've verified by connecting the transformer to a variac and increasing the voltage until I read 1 volt across pins 1 and 6 on the secondary side. The primary impedance is 3000-10,000 CT. The 7189a's put out 24 watts at 400V and have a plate to plate impedance of 8-10,000 ohms. I just am not sure which combination of pins to connect to. I ran 100 vac thru the primary and measured the 15 combinations across the pins on the secondary side and got voltages ranging from .71 for pins 2-3 to 6 across pins 1-6. I'm not sure what to make of these readings or how to go from here.