SSS inspired amp

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Rockwell666
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Joined: Mon Nov 21, 2016 4:56 pm

SSS inspired amp

Post by Rockwell666 »

Howdy folks,

Im building a from scratch SSS style amp. 5xpre 2x6550
Now without coming here before designing (doh), I took a few well documented schematics and put them together.
I took a fender twin reverb preamp added dwell and output controls to the reverb and then used the phase inverter and CF buffer circuit from the fender studio bass. (attached file)
The amp has been built. I got it fired up and so far so good up to the CF buffer circuit. I used a choke and an OT with UL connections. (disconnected right now).
Here is my concern, the B+ for the CF plates is 460VDC!!! I don't want to put in the ouput tubes until I get this sorted out. I feel @ 460VDC it will fry any tube I put in..
I figure I got 2 options:
1) remove the choke, connect the UL taps to the screens and then put in a large voltage divider to get the B+ screen node/CF node (B) to 400vdc.
2) try a 12bh7a in the CF position.

Any other suggestions?
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martin manning
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Re: SSS inspired amp

Post by martin manning »

I think you are ok. With no current draw from the 12AT7 and no output tubes drawing current the voltage will be high. Note in the Studio Bass schematic the voltage on the CF plates is 395. Do you have a Variac to bring it up gradually? If not a lightbulb limiter could be used, working your way up in wattage on the bulb from 40 to start to 100 or 150.
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

martin manning wrote:I think you are ok. With no output tubes drawing current the voltage will be high. Note in the Studio Bass schematic the voltage on the CF plates is 395. Do you have a Variac to bring it up gradually? If not a lightbulb limiter could be used, working your way up in wattage on the bulb from 40 to start to 100 or 150.
I don't have a variac but I do have the light bulb current limiter. GOOD IDEA!! this way I wont destroy things in the tweaking process. I was thinking maybe using some vintage 12au7s and 12bh7s I have laying around before sticking in the new JJs. (infact I was thinking I might need to use a vintage tube for the CF position anyways for reliability).
tictac
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Re: SSS inspired amp

Post by tictac »

Are you reading 460V plate to ground or plate to cathode?

plate to cathode is what the tube is actually seeing.....

TT
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

tictac wrote:Are you reading 460V plate to ground or plate to cathode?

plate to cathode is what the tube is actually seeing.....

TT
Plate to ground. The cathodes should be around 75vdc. So that should get us into better specs.

I am going to fire it up tonight ill post the progress.

THANK YOU GUYS! 8)
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

Okay, so nothing fried :-)

With the current limiter in place im getting 400vdc at the CF plates. Perfect.
I am getting a perfect signal to one output tube grid on one side of the LTP inverter and CF buffer but the other side its not coming through.
The signal is reaching the grids of the LTP but one plate output is off.

The LTP input triode grid is getting 14vdc at max volume the opposite (bottom LTP) grid is seeing 42vdc. Cathode measures 62vdc.
im alittle baffled as to why one side isn't working. I re soldered the connections and couldn't find any cold solders with a DMM. I think maybe a different tube is worth trying. Im also don't have a cap in series between the output level control and the LTP input grid.

The one side that is working, looks great on the scope :-)
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martin manning
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Re: SSS inspired amp

Post by martin manning »

What are the DC volts on the CF pins with no signal?
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

martin manning wrote:What are the DC volts on the CF pins with no signal?
Plates 396vdc

pin2:108vdc
Pin3:152vdc

Pin 7:117vdc
Pin 8:163vdc

using the scope i found the signal stopped from one of (now i forgot) the LTP inverter plate outputs.
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martin manning
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Re: SSS inspired amp

Post by martin manning »

Pins 3 and 8 are supposed to be be 75v, but your high voltage might just be due to tube variation. You probably can't measure 2 and 7 directly since your meter will upset the bias. Instead measure at the bottom of the 2k2 bias resistors and see if the voltage there is about 4-5v below the cathodes. How about the PI's DC voltages, particularly the plates? Again measure the grids at the bottom of the bias resistor. If the plate voltages are reasonable, and at least close, then look for a break in the signal path on the side that isn't working.
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

LTP tube is a new JJ 12ax7
CF tube is an old beat up 12au7

LTP:
pin1:246v
pin2:42v
pin3:65v

pin6:384v
pin7:15.2v
pin8:65v

CF:
pin1:398v
pin2:108v
pin3:153v
bottom of cathode resistor : 143v

pin6:398v
pin7:118v
pin8:164v
bottom of cathode resistor : 153v

I really do appreciate the help guys. I will have some time tonight to tinker more.
The only thing I see that stands out is the plate voltages on the LTP inverter.
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martin manning
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Re: SSS inspired amp

Post by martin manning »

Looks like the CF is ok, but you should put a 12AT7 in there unless you've redesigned for 12AU7.

Something definitely wrong with the PI tube on the 678 triode. Got another 12AX7 to try? As I said above, you can't measure the PI grids directly. Measure at the bottom of the bias resistor (the one connected to 3 and 8).
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

martin manning wrote:Looks like the CF is ok, but you should put a 12AT7 in there unless you've redesigned for 12AU7.

Something definitely wrong with the PI tube on the 678 triode. Got another 12AX7 to try? As I said above, you can't measure the PI grids directly. Measure at the bottom of the bias resistor (the one connected to 3 and 8).
Im on it :-) Thnx Martin
Rockwell666
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its alive

Post by Rockwell666 »

Looks like it was a bad solder somewhere, got it up an running with signals to both output tube grids :-)
Got the 6550's biased up to 45ma and tested the reverb. Everything works great! The only issue I have is the volume is alittle low.
I am going to go through it and check all voltages and signal levels. My guess is too much signal is getting dumped.
also, a crackling noise when flipping the standby. I am going to try a bypass cap on the standby switch to see if that's the issue. I will also get some pics uploaded so you guys can see the beast.

Thnx!!
Rockwell666
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Re: SSS inspired amp

Post by Rockwell666 »

I put a large cap across the standby switch and pulled the CF tube and I am still getting the crackling sound when I flip the standby switch. I am thinking its related to the output tube grids or something in the power supply.
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martin manning
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Re: SSS inspired amp

Post by martin manning »

Rockwell666 wrote:I put a large cap across the standby switch and pulled the CF tube and I am still getting the crackling sound when I flip the standby switch. I am thinking its related to the output tube grids or something in the power supply.
Maybe another bad solder joint? I'd look at the B+ connections and plate loads on all of the tubes. You can divide the power and preamp sections at the loop jacks (assuming you have them) and determine which side it's coming from.

Oops, I see above you said it still crackles with the PI tube out, so yes power stage or power supply. Could be some oscillation breaking out?
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