ampex 620 into modern hifi
Here is a pair of Ampex 620 (or maybe 612's) that I restored, then went past restoration and rebuilt them. (The photo is not of the pair, but they are identical)
The input and output tubes are now triode wired.
I replaced all of the capacitors.
The P.S. electro's were replaced w/ the oil motor run caps that you can see in the photo.
The other caps were replaced w/ nice poly/foil caps.
The input tube is LED bias.
I removed the negative feedback
I think that's about all I ended up doing. Maybe I'll add a CCS to the LTP sometime in the future.
I kept all of the original parts, and the circuit could be made stock if these ever become collectors items or something. It however sounds much better like this
.
Thanks to everyone here for their help. I can post before and after schematics, if anyone cares.
Here is a pair of Ampex 620 (or maybe 612's) that I restored, then went past restoration and rebuilt them. (The photo is not of the pair, but they are identical)
The input and output tubes are now triode wired.
I replaced all of the capacitors.
The P.S. electro's were replaced w/ the oil motor run caps that you can see in the photo.
The other caps were replaced w/ nice poly/foil caps.
The input tube is LED bias.
I removed the negative feedback
I think that's about all I ended up doing. Maybe I'll add a CCS to the LTP sometime in the future.
I kept all of the original parts, and the circuit could be made stock if these ever become collectors items or something. It however sounds much better like this
.Thanks to everyone here for their help. I can post before and after schematics, if anyone cares.
Attachments
New 300B P-P Monoblocks
Here is a new pair of 300B push-pull monoblocks. They are UTC LS-30 repeat coil balanced input to ECC99, LL1667/5 choke coupled to 300Bs or JJ2A3-40s. UTC LS-54 OPT connected for 12K5:8R by using the 5 ohm tap.
Currently one is running 300Bs and the other 2A3-40s and they are identical in op point, gain, power, and sound!
400V plate-cathode, 45mA Iq, -84V grid-cathode. I'm getting about 14 watts continuous sinewave power at clipping. Frequency response +/- 1dB from <10Hz to >30 KHz, then peaks +2dB at 50KHz, peaks again +5dB at 70 KHz. With no NFB I guess the HF peaks are expected and should probably add some compensation...
They sound great and have plenty of power for my 94dB/W speakers. I'll do some more measurement after a while but have been enjoying listening to them too much.
Cheers,
Michael
Here is a new pair of 300B push-pull monoblocks. They are UTC LS-30 repeat coil balanced input to ECC99, LL1667/5 choke coupled to 300Bs or JJ2A3-40s. UTC LS-54 OPT connected for 12K5:8R by using the 5 ohm tap.
Currently one is running 300Bs and the other 2A3-40s and they are identical in op point, gain, power, and sound!
400V plate-cathode, 45mA Iq, -84V grid-cathode. I'm getting about 14 watts continuous sinewave power at clipping. Frequency response +/- 1dB from <10Hz to >30 KHz, then peaks +2dB at 50KHz, peaks again +5dB at 70 KHz. With no NFB I guess the HF peaks are expected and should probably add some compensation...
They sound great and have plenty of power for my 94dB/W speakers. I'll do some more measurement after a while but have been enjoying listening to them too much.
Cheers,
Michael
Attachments
300B Push-pull schematic
Here is the signal path schematic. The driver is DC coupled through the LL1667/5mA anode choke which has 1200 ohms DCR per side. The 300Bs operate mostly in self-bias against a fairly stable -18V on the grids. the shunt-regulated drive stage is stable enough to settle within about 0.5 V side to side.
Cheers,
Michael
Here is the signal path schematic. The driver is DC coupled through the LL1667/5mA anode choke which has 1200 ohms DCR per side. The 300Bs operate mostly in self-bias against a fairly stable -18V on the grids. the shunt-regulated drive stage is stable enough to settle within about 0.5 V side to side.
Cheers,
Michael
Attachments
Re: Beautiful work, Michael!
Thanks for the comments.
I should have been more specific. The LS-54 is 8K anode-anode with various output taps from 2.5R to 30R. I use the 5R output tap to drive an 8 ohm load, which provides the output tubes about 12K5 anode to anode. This is about 6250 ohms load on each 300B. This is a much higher value than usual, but I like the sound better than using the 7.5 ohm tap, and the power output decrease ifrom 18 watts to 14 watts is really not much, volume-wise. Of course, the signal voltage swing + DC imbalance saturating the OPT primary is still a limiting factor, but the LS-54 seems to be working great at 10Hz and 14 watts output.
The WE chart for 300B recommended operating conditions gives plate load impedances up to 6500 ohms. I guess 5K per anode, 10K anode-anode would be my first choice but the 12K5 is good enough for now. I can't find my copy of "the chart" right now, but suffice to say that 300Bs run great at higher load impedance and lower standing current as a design alternative to the 3K/60mA per anode frequently seen.The published characteristic curves only go to -140 grid voltage but the tube is linear with signal swing from 0 to -200 grid-cathode (and beyond...)
Cheers,
Michael
tomsyl said:
Thanks for the comments.
I should have been more specific. The LS-54 is 8K anode-anode with various output taps from 2.5R to 30R. I use the 5R output tap to drive an 8 ohm load, which provides the output tubes about 12K5 anode to anode. This is about 6250 ohms load on each 300B. This is a much higher value than usual, but I like the sound better than using the 7.5 ohm tap, and the power output decrease ifrom 18 watts to 14 watts is really not much, volume-wise. Of course, the signal voltage swing + DC imbalance saturating the OPT primary is still a limiting factor, but the LS-54 seems to be working great at 10Hz and 14 watts output.
The WE chart for 300B recommended operating conditions gives plate load impedances up to 6500 ohms. I guess 5K per anode, 10K anode-anode would be my first choice but the 12K5 is good enough for now. I can't find my copy of "the chart" right now, but suffice to say that 300Bs run great at higher load impedance and lower standing current as a design alternative to the 3K/60mA per anode frequently seen.The published characteristic curves only go to -140 grid voltage but the tube is linear with signal swing from 0 to -200 grid-cathode (and beyond...)
Cheers,
Michael
RCA BA-4C Monitor Amplifiers
I found two of these RCA BA-4C Monitor Amplifiers while cleaning out some storage spaces. Two 6L6G's. They are supposed to be used with the RCA rackmount shelves into which they slide and latch, as the plugs on the back fit into sockets. I have not found the shelves. I think these are from the late 1940's. Also found was one BX-1D pre-amp power supply but no preamp found yet..
I found two of these RCA BA-4C Monitor Amplifiers while cleaning out some storage spaces. Two 6L6G's. They are supposed to be used with the RCA rackmount shelves into which they slide and latch, as the plugs on the back fit into sockets. I have not found the shelves. I think these are from the late 1940's. Also found was one BX-1D pre-amp power supply but no preamp found yet..
At last, the final touch and voilá, Russian roulette!!
6P14P in SE, triode-strapped with a single 6N2P for driver.
Schematic will be found at: http://www.joddla.net/audio/6p14pse/el84sexun_jpg.JPG
6P14P in SE, triode-strapped with a single 6N2P for driver.
Schematic will be found at: http://www.joddla.net/audio/6p14pse/el84sexun_jpg.JPG
Attachments
Re: Beautiful work, Michael!
I can only say, "why did you have them make them then?" Seriously? I am completely against nOObs thinking they should custom wide iron. No. NO. NOO. There is a wonderful array of iron out there... for all tubes, especially the 300b. Come on now... why wind you own when you work at the Mall, Wendys, Paff and Smith, etc... when you should focus on your amp, not ridiculous custom wound transformers that you have no idea what-so-ever will perform like?
tomsyl said:
I can only say, "why did you have them make them then?" Seriously? I am completely against nOObs thinking they should custom wide iron. No. NO. NOO. There is a wonderful array of iron out there... for all tubes, especially the 300b. Come on now... why wind you own when you work at the Mall, Wendys, Paff and Smith, etc... when you should focus on your amp, not ridiculous custom wound transformers that you have no idea what-so-ever will perform like?