Looking for comments on modifying my WE KS-16617-L1 monoblocks to use the low-gain side as a preamp. These amplifiers combine a 12AU7 primary stage with push-pull 6V6 design for high gain, along with a relay-activated low-gain side (6BA6/12AT7) that was used to amplify a condenser microphone to headset/loudspeaker. Could the low-gain output be made suitable as a preamp circuit?
The low-gain works input: T3/R3 output: T4/R4B.
The high-gain works input: T1/R1 output: T/R or T2/R2.
Attached is the schematic.
The low-gain works input: T3/R3 output: T4/R4B.
The high-gain works input: T1/R1 output: T/R or T2/R2.
Attached is the schematic.
Always worth doing the basics such as secondary side fuse, cathode bias senses, grid and screen stoppers for the output stages.
There are enough preamp valves to effectively have 4 triodes available for effects, as a pentode would be enough to drive the master vol. You could even mave the master a gain, and add a master vol before the PI driver.
There are low pass filters in there that would typically restrict bandwidth to telephony voice, but be a bit heavy for guitar.
Have you worked out the OT output windings yet?
There are enough preamp valves to effectively have 4 triodes available for effects, as a pentode would be enough to drive the master vol. You could even mave the master a gain, and add a master vol before the PI driver.
There are low pass filters in there that would typically restrict bandwidth to telephony voice, but be a bit heavy for guitar.
Have you worked out the OT output windings yet?
Right now, I am using the High-Level Gain side (T1-R1) as a line level input and agree that it seems weak on the higher frequencies.
trobbins, the only info I have on the output is from the documentation: T-R = 4 ohms (nominal rated load), internal impedance 1 ohm, T5-R5 = 1 ohm (nominal rated load), internal impedance 0.3 ohm, T2-R2 = 600 ohms or 70.7 volt distribution line, internal impedance 150 ohm
nec3, I really just wanted to improved the amp for hi-fi listening. Can you recommend some modifications? Not using the Low Level side, so I don't think that the input transformer affects my listening.
BTW, thanks for the help. I am really learning quite a bit from this site.
trobbins, the only info I have on the output is from the documentation: T-R = 4 ohms (nominal rated load), internal impedance 1 ohm, T5-R5 = 1 ohm (nominal rated load), internal impedance 0.3 ohm, T2-R2 = 600 ohms or 70.7 volt distribution line, internal impedance 150 ohm
nec3, I really just wanted to improved the amp for hi-fi listening. Can you recommend some modifications? Not using the Low Level side, so I don't think that the input transformer affects my listening.
BTW, thanks for the help. I am really learning quite a bit from this site.
So it sounds like the OT is useable for a speaker without having to investigate other winding configurations.
Are you able to spot the added RC circuits that are purposefully limiting the amps bandwidth? There are a few, and you can reverse engineer, or remove, them to give nominal 15kHz roll-offs. Usually the change needed is to lower the capacitance in the filter.
Are you able to spot the added RC circuits that are purposefully limiting the amps bandwidth? There are a few, and you can reverse engineer, or remove, them to give nominal 15kHz roll-offs. Usually the change needed is to lower the capacitance in the filter.
From your first post i thought you wanted to use its low level side as a pre-amplifier..
This file may be useful :
http://doc.telephonecollectors.info/dm/024-150-504_I1.pdf
It says band width of the high level side is 50-10k c/s which is actually not too bad.
To improve it even better,
1. You can bypass cathode rezistors R22, R18 with 100 uF for each connected across them.
2. As Trobbins said, replace C11, C16, C17 with a 100pF.
The output transformer is the critical part for band width as also most expensive. If the improvement after above changes makes you happy with sound, you can leave it alone.
Do you have Audio analyzer ?
This file may be useful :
http://doc.telephonecollectors.info/dm/024-150-504_I1.pdf
It says band width of the high level side is 50-10k c/s which is actually not too bad.
To improve it even better,
1. You can bypass cathode rezistors R22, R18 with 100 uF for each connected across them.
2. As Trobbins said, replace C11, C16, C17 with a 100pF.
The output transformer is the critical part for band width as also most expensive. If the improvement after above changes makes you happy with sound, you can leave it alone.
Do you have Audio analyzer ?
nec3 - no audio analyzer and you were right about the first post. When I read your response I realized that the Low-Level side was designed for voice bandwidth, my focus moved to the High Level side.
Basically inherited a bunch of WE stuff from granddad and these are the only two complete amps. They are in perfect condition and I was grinning ear to ear with the sound when I finished going over them and fitted them with new tubes, but I think finding this site has made me wonder if they can be better(?) I guess this is how it all begins ;-)
I will take your recommendations on the cathode resistor bypassing, but I am curious about the power stage capacitors. Any recommendations on sourcing these? Stock rating on C16 and C17 is 1600v and I haven't found a 100pF capacitor in this pile yet. Everything I see online is disc and the original ones are Cornell-Dubilier axial type (poly?). Won't this affect the sound quality using ceramic?
Basically inherited a bunch of WE stuff from granddad and these are the only two complete amps. They are in perfect condition and I was grinning ear to ear with the sound when I finished going over them and fitted them with new tubes, but I think finding this site has made me wonder if they can be better(?) I guess this is how it all begins ;-)
I will take your recommendations on the cathode resistor bypassing, but I am curious about the power stage capacitors. Any recommendations on sourcing these? Stock rating on C16 and C17 is 1600v and I haven't found a 100pF capacitor in this pile yet. Everything I see online is disc and the original ones are Cornell-Dubilier axial type (poly?). Won't this affect the sound quality using ceramic?
I do not think you can find "Cornell-Dubilier " caps. But any cap under 500pF with same voltage rating will be good.
"these are the only two complete amps. They are in perfect condition"
Then i would try my best to keep this two original and make one using the others which are in bad condition.
It is easy to buy/make a new tube amplifier but not always easy to find old original ones such as theese two WE.
C7 is very severe at 1.5kHz in the preamp, and the preamp is set up for a mic input with some loading on the preamp output transformer via what appears to be an agc or limiter circuit to alter V1 bias, which is possibly not what you want?
A normal preamp circuit would effectively disconnect that related circuitry.
The power amp RC networks work out to be not too excessive for guitar type application, but may be noticeable for hi-fi audio.
The 6V6 anode RC loads add a very heavy 1kohm load above 53kHz, which will start to be an influence above 5kHz. I would recommend removing those 2 networks, and adding protection MOVs across the OT half primary windings for in-audible protection. If the networks were left, then the value chosen would depend on your application - eg. 1NF caps of 3kV would be appropriate for a guitar or basic hi-fi amp. Similarly for the C20 R11, which has a 33kHz corner.
A normal preamp circuit would effectively disconnect that related circuitry.
The power amp RC networks work out to be not too excessive for guitar type application, but may be noticeable for hi-fi audio.
The 6V6 anode RC loads add a very heavy 1kohm load above 53kHz, which will start to be an influence above 5kHz. I would recommend removing those 2 networks, and adding protection MOVs across the OT half primary windings for in-audible protection. If the networks were left, then the value chosen would depend on your application - eg. 1NF caps of 3kV would be appropriate for a guitar or basic hi-fi amp. Similarly for the C20 R11, which has a 33kHz corner.
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