I may be getting a Heathkit A9 in decent condition with no tubes and a replaced power transformer soon.
I looked at the schematic and given my experience with tube amps and recently discovering how easy it is to do fixed bias instead of cathode bias, I may do that as it will improve the bass response.
Now in order to do another mod which is to increase the B+ cap value the output transformer connects to so that the amp can handle the bass better (did that mod and the fixed bias mod to a Magnavox AMP-142 and the sound quality improved a good bit) I'd have to go with a solid state power supply unless I go with a higher voltage power transformer, however losing the 5U4 will reduce the heat the amp puts out.
I can either use a lower voltage power transformer (may be replacing it anyways depending on what was stuck in there in place of the original) and a solid state supply or I can use the same voltage and add a filter cap after the diodes and a choke between the first and second filter caps to drop the voltage some and provide better filtering which will also reduce the initial surge current of the larger filter cap.
How much B- voltage should I have on the 6L6 control grids and what idle cathode current should I run them at?
That said if I'm reading the schematic right, there's no cap across the 6L6 cathode resistor. So one benefit of fixed bias which is getting rid of that pesky cathode bypass cap will not be realized so do I really need to mod it for fixed bias?
Now I've tested amps before with no cathode bypass cap and if I remember correctly that wasn't always the best option far as audio quality goes. I forget exactly what problem I saw with an amp that had no cathode bypass cap on the push pull output tube cathode resistor, but I know there was some issue.
I do know that in order to limit the effect of Miller capacitance I will need to replace the 1 meg volume control with a 100k volume control and I'll up the value of other coupling caps in the amp to reduce the phase shift at 20Hz to near zero as that will also improve the sound quality plus eliminate any potential bass instability issues caused by the phase shift. My way of thinking is if the phase shift is not there, it cannot cause problems.
As I typically do, I'll restore this amp to stock before making those two major mods just to see how well the amp works. That way I have a good baseline of what to expect from the amp.
EDIT:
It looks like I can maybe connect the line level input straight to the volume control as the tuner input uses a 1meg series resistor that connects to a 18k ressitor which goes to ground. So it attenuates the signal pretty heavily then steps it back up with the 12AX7.
Now if I do that and it works I'll install a 1/4" switched jack so that the original amp inputs can still function as designed when nothing is plugged into the jack.
I looked at the schematic and given my experience with tube amps and recently discovering how easy it is to do fixed bias instead of cathode bias, I may do that as it will improve the bass response.
Now in order to do another mod which is to increase the B+ cap value the output transformer connects to so that the amp can handle the bass better (did that mod and the fixed bias mod to a Magnavox AMP-142 and the sound quality improved a good bit) I'd have to go with a solid state power supply unless I go with a higher voltage power transformer, however losing the 5U4 will reduce the heat the amp puts out.
I can either use a lower voltage power transformer (may be replacing it anyways depending on what was stuck in there in place of the original) and a solid state supply or I can use the same voltage and add a filter cap after the diodes and a choke between the first and second filter caps to drop the voltage some and provide better filtering which will also reduce the initial surge current of the larger filter cap.
How much B- voltage should I have on the 6L6 control grids and what idle cathode current should I run them at?
That said if I'm reading the schematic right, there's no cap across the 6L6 cathode resistor. So one benefit of fixed bias which is getting rid of that pesky cathode bypass cap will not be realized so do I really need to mod it for fixed bias?
Now I've tested amps before with no cathode bypass cap and if I remember correctly that wasn't always the best option far as audio quality goes. I forget exactly what problem I saw with an amp that had no cathode bypass cap on the push pull output tube cathode resistor, but I know there was some issue.
I do know that in order to limit the effect of Miller capacitance I will need to replace the 1 meg volume control with a 100k volume control and I'll up the value of other coupling caps in the amp to reduce the phase shift at 20Hz to near zero as that will also improve the sound quality plus eliminate any potential bass instability issues caused by the phase shift. My way of thinking is if the phase shift is not there, it cannot cause problems.
As I typically do, I'll restore this amp to stock before making those two major mods just to see how well the amp works. That way I have a good baseline of what to expect from the amp.
EDIT:
It looks like I can maybe connect the line level input straight to the volume control as the tuner input uses a 1meg series resistor that connects to a 18k ressitor which goes to ground. So it attenuates the signal pretty heavily then steps it back up with the 12AX7.
Now if I do that and it works I'll install a 1/4" switched jack so that the original amp inputs can still function as designed when nothing is plugged into the jack.
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Good news. Just got pictures of the amp and it isn't near as bad looking as the description sounded.
Whoever replaced the power transformer did an excellent job.
One knob is broken in one spot, but I'll swap the volume knob with it and use a different knob on the volume control which I may have to do anyways when I switch it to a 100k pot.
I hate drilling a hole in the chassis for a 1/4" jack, but it's already been mucked up some by the replacement power transformer.
Just looked at the photo of the front panel and it's actually the A9C.
Whoever replaced the power transformer did an excellent job.
One knob is broken in one spot, but I'll swap the volume knob with it and use a different knob on the volume control which I may have to do anyways when I switch it to a 100k pot.
I hate drilling a hole in the chassis for a 1/4" jack, but it's already been mucked up some by the replacement power transformer.
Just looked at the photo of the front panel and it's actually the A9C.
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I could, but I need a 1/4" switched jack as I want to feed the line input to the volume control given how much the signal level is being reduced by the resistors on the line input.
There is a 1 meg series resistor and a 18k resistor after it to ground.
For the specified 600mVrms input that gives me an output of 11mVrms.
I think reducing the line level input that much only to amplify it by the preamp stage is dumb as it could potentially reduce the audio quality.
Now if I do that, the output will be inverted phase given I'll be bypassing one 12AX7 stage which might not be a good idea.
What I'll do is restore the amp to 100% stock then thoroughly test it before deciding if any mod needs to be done.
I will definitely be changing the stock 1 meg volume control to a 100k volume control given the 1 meg pot will cause miller capacitance issues which will reduce the treble some unless the amp compensates for that.
A frequency response test will determine how flat the amp is. I'll run the test with the volume control near full CCW, mid position and full CW to get an idea what if any treble is being lost due to miller capacitance effects with the stock volume control.
I will also do a low frequency response test to see how much if any phase shift is present. I know some coupling caps may need to be increased in size to eliminate low frequency phase shift, but the test will reveal which ones to up in value.
Now the 18k resistor after the 1 meg resistor on the line input will keep the second 12AX7 stage from having miller capacitance effects.
There is a 1 meg series resistor and a 18k resistor after it to ground.
For the specified 600mVrms input that gives me an output of 11mVrms.
I think reducing the line level input that much only to amplify it by the preamp stage is dumb as it could potentially reduce the audio quality.
Now if I do that, the output will be inverted phase given I'll be bypassing one 12AX7 stage which might not be a good idea.
What I'll do is restore the amp to 100% stock then thoroughly test it before deciding if any mod needs to be done.
I will definitely be changing the stock 1 meg volume control to a 100k volume control given the 1 meg pot will cause miller capacitance issues which will reduce the treble some unless the amp compensates for that.
A frequency response test will determine how flat the amp is. I'll run the test with the volume control near full CCW, mid position and full CW to get an idea what if any treble is being lost due to miller capacitance effects with the stock volume control.
I will also do a low frequency response test to see how much if any phase shift is present. I know some coupling caps may need to be increased in size to eliminate low frequency phase shift, but the test will reveal which ones to up in value.
Now the 18k resistor after the 1 meg resistor on the line input will keep the second 12AX7 stage from having miller capacitance effects.
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What I may do is this.
If the amp reduces the treble some and I find the input stage is causing it, I'll just disconnect the line in jack and the 12AX7 output, connect the line in jack to the coupling cap that went to the 12AX7 plate and reverse the speaker wire connections at the output. That way I add no extra holes and it can be easily reversed. I'll also update the schematic to reflect that change.
Is there a cleaner schematic than what I found available online in a brief search, or is that the best one available for download?
Also it looks like maybe Heathkit knew the issues cathode bypass caps cause so eliminated them everywhere except the one spot where the higher gain was needed.
Also given the 560 ohm resistor right after the rectifier, I can easily up the value of the first filter cap to 200uF no problem. Only reason I can figure for that resistor being there is it reduces the B+ some which may have been done when they switched to the 5V4 rectifier given it drops less voltage than the 5U4.
If the amp reduces the treble some and I find the input stage is causing it, I'll just disconnect the line in jack and the 12AX7 output, connect the line in jack to the coupling cap that went to the 12AX7 plate and reverse the speaker wire connections at the output. That way I add no extra holes and it can be easily reversed. I'll also update the schematic to reflect that change.
Is there a cleaner schematic than what I found available online in a brief search, or is that the best one available for download?
Also it looks like maybe Heathkit knew the issues cathode bypass caps cause so eliminated them everywhere except the one spot where the higher gain was needed.
Also given the 560 ohm resistor right after the rectifier, I can easily up the value of the first filter cap to 200uF no problem. Only reason I can figure for that resistor being there is it reduces the B+ some which may have been done when they switched to the 5V4 rectifier given it drops less voltage than the 5U4.
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The amp is on its way to me.
I don't have enough spare tubes to retube the amp as it's coming without tubes so I will have to order them.
Probably gonna buy them off the Antique Radio Forums like I did the tubes for my Magnavox AMP-142.
When I get the amp I'll measure the power transformer voltages and then I'll see if Edcor has one with the proper specs and a bias tap so that I can go fixed bias.
I had thought of installing a milliammeter in the cathode circuit of the 6L6 tubes, but I may buy one of these instead
https://www.amazon.com/Nobsound-Cur...n/dp/B0BYMK2XCM/ref=sr_1_1?crid=1Y9I0JI2WFYC1
and leave it plugged in permanently.
That way I don't have to alter the chassis to mount a meter and I can use two pots to adjust the cathode current of each 6L6 so that I don't have to buy a matched pair. The only thing then required is two small holes for the trimpots unless I mount them under the chassis. What I'll then do is add a 1 ohm resistor between each cathode and ground so that I still have a way to measure bias if I decide not to always leave the meters in circuit.
Also what is the general consensus about using a small four band analog EQ such as the Schiit Loki Mini+ https://www.amazon.com/Nobsound-Cur...n/dp/B0BYMK2XCM/ref=sr_1_1?crid=1Y9I0JI2WFYC1 before the amp to give more control over the tone and also to insure the amp sees a low impedance source (75 ohms) to eliminate miller capacitance issues?
I don't have enough spare tubes to retube the amp as it's coming without tubes so I will have to order them.
Probably gonna buy them off the Antique Radio Forums like I did the tubes for my Magnavox AMP-142.
When I get the amp I'll measure the power transformer voltages and then I'll see if Edcor has one with the proper specs and a bias tap so that I can go fixed bias.
I had thought of installing a milliammeter in the cathode circuit of the 6L6 tubes, but I may buy one of these instead
https://www.amazon.com/Nobsound-Cur...n/dp/B0BYMK2XCM/ref=sr_1_1?crid=1Y9I0JI2WFYC1
and leave it plugged in permanently.
That way I don't have to alter the chassis to mount a meter and I can use two pots to adjust the cathode current of each 6L6 so that I don't have to buy a matched pair. The only thing then required is two small holes for the trimpots unless I mount them under the chassis. What I'll then do is add a 1 ohm resistor between each cathode and ground so that I still have a way to measure bias if I decide not to always leave the meters in circuit.
Also what is the general consensus about using a small four band analog EQ such as the Schiit Loki Mini+ https://www.amazon.com/Nobsound-Cur...n/dp/B0BYMK2XCM/ref=sr_1_1?crid=1Y9I0JI2WFYC1 before the amp to give more control over the tone and also to insure the amp sees a low impedance source (75 ohms) to eliminate miller capacitance issues?
The beauty of tubes for me is the simplicity; a handful of passive components, a couple of tubes and good old fashioned iron, transformers that look identical to the one from 100 years ago. The result is music that is rendered in some way that makes it more interesting to listen to. For the same reason we listen to an organ in the church instead of in an open field, the colourations are part of the involvement. In a PP amp there is less distortion, but still there is a soundstage somehow, probably due to minute phase differences in the music that create 3D detail to our ears.
So, the equalizer would not be for me, but maybe you need it to enjoy your amplifier.
So, the equalizer would not be for me, but maybe you need it to enjoy your amplifier.
The reason why I comment a bit about this topic because I also have an old Pioneer amplifier and am a bit conflicted how I should fix it to make it useful. Fortunately my amp is tatty and does not have a large fanbase, so I am at liberty to try things with it without worrying about its resale value. I have hit a dead end with that project and it sits on the desk while I decide whether I fix its suboptimal RIAA stage and all the controls and switches for bells and whistles', or strip out the bits that affect the sonic qualities. Ultimately it will never be as good as your amp, for instance.
In my case I will look at an option to 'defeat' the tone stage, to remove an unnecessary stage IMO. The danger with your A9 is that the tone controls get in the way of the music because they were more for a different world where there were no digital sources, just scratchy, rumbly analogue sources.
In my case I will look at an option to 'defeat' the tone stage, to remove an unnecessary stage IMO. The danger with your A9 is that the tone controls get in the way of the music because they were more for a different world where there were no digital sources, just scratchy, rumbly analogue sources.
I only use tone controls as necessary.
For me most amps with tone controls sound great with the controls set mid position.
I also have a few amps with no tone controls that sound great as well.
If your Pioneer is tube, there's a few things you can do to perhaps improve its sound.
Maybe you can start a topic about it on the forums here and I'll give you some advice as to what can be done to your amp.
For me most amps with tone controls sound great with the controls set mid position.
I also have a few amps with no tone controls that sound great as well.
If your Pioneer is tube, there's a few things you can do to perhaps improve its sound.
Maybe you can start a topic about it on the forums here and I'll give you some advice as to what can be done to your amp.
These two for starters ...
Pioneer SM-R150
LTSpice RIAA stage shootout
And on this general thread ...
Restoration of Golden Age Amplifiers
As you can see from the last thread there is not so much appetite on this forum for arcane equipment. I usually go to the UK Vintage Radio forum if I need to talk about a specific piece of old equipment.
Pioneer SM-R150
LTSpice RIAA stage shootout
And on this general thread ...
Restoration of Golden Age Amplifiers
As you can see from the last thread there is not so much appetite on this forum for arcane equipment. I usually go to the UK Vintage Radio forum if I need to talk about a specific piece of old equipment.
The amp arrived today.
Whoever replaced the power transformer also did some other stuff to the amp such as shorting the 560 ohm resistor between the 5V4 and first filter cap and moving the feedback connection to the 8 ohm transformer tap.
Not sure what else has been done so this is going to be a case of ordering all parts per the schematic and verifying the wiring is 100% correct before powering it up.
What I wonder is if the power transformer is a little lower in HV and maybe the resistor was shorted to make the B+ correct.
That said two filter caps (one under chassis one can cap) has stuff that oozed out of the vent in the bottom of the cans.
So there's no telling what happened. Either the transformer was replaced before the caps gave out and age got to them or they gave out after the transformer was replaced due to the B+ exceeding their rating.
I'll need a 5V4, two 6L6 a 12AX7 and a 12AU7. I think I have a spare top for the shielded 12AX7 socket.
I have two 6L6 WGB I'll use as the console those tubes came from isn't being used at the moment and I'll order two for it. I know that one can use regular 6L6 tubes so thats what I'll get for it.
This amp will be used more so the 6L6 WGB may last longer.
Whoever replaced the power transformer also did some other stuff to the amp such as shorting the 560 ohm resistor between the 5V4 and first filter cap and moving the feedback connection to the 8 ohm transformer tap.
Not sure what else has been done so this is going to be a case of ordering all parts per the schematic and verifying the wiring is 100% correct before powering it up.
What I wonder is if the power transformer is a little lower in HV and maybe the resistor was shorted to make the B+ correct.
That said two filter caps (one under chassis one can cap) has stuff that oozed out of the vent in the bottom of the cans.
So there's no telling what happened. Either the transformer was replaced before the caps gave out and age got to them or they gave out after the transformer was replaced due to the B+ exceeding their rating.
I'll need a 5V4, two 6L6 a 12AX7 and a 12AU7. I think I have a spare top for the shielded 12AX7 socket.
I have two 6L6 WGB I'll use as the console those tubes came from isn't being used at the moment and I'll order two for it. I know that one can use regular 6L6 tubes so thats what I'll get for it.
This amp will be used more so the 6L6 WGB may last longer.
Haven't seen that discussed much...
But my thinking was same as yours. We all have different sound tastes, acoustical environments, and speaker types. I added 7 band EQ to front end of my PP amp and will do it again in future designs-
https://www.aliexpress.us/item/3256805001763519.html?
Some may disagree- perhaps opinion that it takes away from a pure 'tube sound', but imo- to each his listening own
Jim
Attachments
That board is interesting in that it has two sets of inputs and outputs. I'm assuming the inputs are in parallel as well as the outputs.
Indeed some think it takes away from the pure tube sound, however I bet most of them are feeding their tube amp with a DAC or other source that is solid state unless all they listen to is a tube tuner, reel to reel with tubes in it and/or or a tube phono preamp.
Also some are against tone controls to begin with so adding a box with more of them is not what they prefer doing.
I used the Schiit Saga S preamp with a Magnavox AMP 142 and initially I used the active FET stage. I then built a tube buffer with some gain (single 12AU7 1/2 a gain stage the other 1/2 a cathode follower) as the Magnavox needed a higher input signal than what the gain of one preamp could provide and I switched the preamp to passive at that time. Could hear no audible difference whatsoever between the active and passive mode of the preamp.
That said one reason for using the Schiit is the low output impedance it has which will be better for the amp as some tube amps when fed with a higher impedance source (vintage tube tuner for instance) will have miller capacitance issues with the first stage (causes a reduction in the upper treble) if it's a triode that has a plate resistor of greater than 100k.
I did find a real legible schematic with the voltages listed which is nice. Only thing I am unsure of is what output transformer tap the feedback resistor connects to.
Indeed some think it takes away from the pure tube sound, however I bet most of them are feeding their tube amp with a DAC or other source that is solid state unless all they listen to is a tube tuner, reel to reel with tubes in it and/or or a tube phono preamp.
Also some are against tone controls to begin with so adding a box with more of them is not what they prefer doing.
I used the Schiit Saga S preamp with a Magnavox AMP 142 and initially I used the active FET stage. I then built a tube buffer with some gain (single 12AU7 1/2 a gain stage the other 1/2 a cathode follower) as the Magnavox needed a higher input signal than what the gain of one preamp could provide and I switched the preamp to passive at that time. Could hear no audible difference whatsoever between the active and passive mode of the preamp.
That said one reason for using the Schiit is the low output impedance it has which will be better for the amp as some tube amps when fed with a higher impedance source (vintage tube tuner for instance) will have miller capacitance issues with the first stage (causes a reduction in the upper treble) if it's a triode that has a plate resistor of greater than 100k.
I did find a real legible schematic with the voltages listed which is nice. Only thing I am unsure of is what output transformer tap the feedback resistor connects to.
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