I would like to repurpose this small 6BQ5 SE amp out of an old record player. I do not care to use it for phono, plan to hook a BT receiver to it and go. I have no "Grand Illusion" as to the fidelity, tone stack or volume controls are not required. I will recap it and check resistor values that they haven't drifted. The purpose for it for a small office.
1. Bypassing the tone stack - I have tried it using just the second gain stage and it wasn't enough to drive the power tube. Conversely cascading the 2 halves of the of the 12AX7 was way too much gain, 12AV7 sounded about right.
Recommendations on bypassing the TS and volume control? BTW the controls are on a long umbilical.
Any recommendations are appreciated.
BL
1. Bypassing the tone stack - I have tried it using just the second gain stage and it wasn't enough to drive the power tube. Conversely cascading the 2 halves of the of the 12AX7 was way too much gain, 12AV7 sounded about right.
Recommendations on bypassing the TS and volume control? BTW the controls are on a long umbilical.
Any recommendations are appreciated.
BL
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The 6BQ5 screen is connected to . . . what?
B+?
Ultra Linear Tap?
Plate?
Thanks!
A 12AX7 is a low(er) plate current tube
Relatively, a 12AV7 and a 12AY7:
A 12AV7 is more linear at high(er) plate current
A 12AY7 is more linear at low(er) plate current
The stock resistances in the amplifier, perhaps they are more suited for a 12AY7, than a 12AV7.
B+?
Ultra Linear Tap?
Plate?
Thanks!
A 12AX7 is a low(er) plate current tube
Relatively, a 12AV7 and a 12AY7:
A 12AV7 is more linear at high(er) plate current
A 12AY7 is more linear at low(er) plate current
The stock resistances in the amplifier, perhaps they are more suited for a 12AY7, than a 12AV7.
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Here's the Sams version of the schematic.
I'm surprised that you can't get enough output from just bypassing the first 12AX7. What source are you using?
A while back I modded a Voice of Music amp that is also SE 6BQ5. It had a totally separate preamp section so just the two 6BQ5s on the main chassis. I modded it to use cathode feedback so I could use it with a variety of separate preamps.
To my surprise, it actually can function as a spud with a line level signal and no input tubes at all. It certainly has more output and sounds better with a preamp but it's perfectly usable without one, especially as a desktop amp listening nearfield.
I'm surprised that you can't get enough output from just bypassing the first 12AX7. What source are you using?
A while back I modded a Voice of Music amp that is also SE 6BQ5. It had a totally separate preamp section so just the two 6BQ5s on the main chassis. I modded it to use cathode feedback so I could use it with a variety of separate preamps.
To my surprise, it actually can function as a spud with a line level signal and no input tubes at all. It certainly has more output and sounds better with a preamp but it's perfectly usable without one, especially as a desktop amp listening nearfield.
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A single 12AX7 triode driver stage and a 6BQ5 in Beam Power mode . . .
I am not surprised that it has plenty of gain for near field listening.
I am not surprised that it has plenty of gain for near field listening.
The OP claims his doesn't have enough with a single 12AX7.
Mine works, although not optimally, with just a signal going into the 6BQ5s, no input tube. Hence my confusion.
Maybe that 6BQ5 is weak. It often is, on old unrestored tube radios and record players. If the tube is good, increasing the value of the feedback resistor from 68K to 100K may be a quick fix.
Chuck thanks for the proper schematic, I’ll check voltages and component values to see why a single 12AX7 doesn’t have the drive it should. May have some leaky coupling caps. I’ll check tomorrow. BTW, any thoughts on the original question?
BL
BL
Sure, just move your inputs to the grids of the second 12AX7 - the points marked 1 and 4 on the schematic. I assume you have a way to control the volume at the source.
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So took voltages and I swapped out a the coupling caps after the second gain stage that seemed marginal, swapped tubes and "voila'". I also had to remind myself "I have no "Grand Illusion" as to the fidelity". Although it doesn't sound too bad.
Now I am wondering (dangerous!!!) if to use just the second gain stage and call it good or if there is a way to use both???
This chassis is copper clad so I am thinking about stripping the components off of it and cleaning/polishing it up and rebuilding it or don't mess with the years of service and letting it continue to age.
Here are the voltages, all look good per schematic except power tube plate and grid.
Now I need to mosey on over to the speaker section and ask if there is anything that these speakers can go in that will make them useful for this little amp.😎
Thank you for the help and comments.
BL
Now I am wondering (dangerous!!!) if to use just the second gain stage and call it good or if there is a way to use both???
This chassis is copper clad so I am thinking about stripping the components off of it and cleaning/polishing it up and rebuilding it or don't mess with the years of service and letting it continue to age.
Here are the voltages, all look good per schematic except power tube plate and grid.
Now I need to mosey on over to the speaker section and ask if there is anything that these speakers can go in that will make them useful for this little amp.😎
Thank you for the help and comments.
BL
Attachments
Using the first stage would give you much increased sensitivity - so a very small input could be used.
The second stage's grid would need more of a "line level" drive.
A simple volume pot is a nice thing to have though, to tweak the amp's overall level.
The second stage's grid would need more of a "line level" drive.
A simple volume pot is a nice thing to have though, to tweak the amp's overall level.