Quad II input impedance
Howdy, I'm wondering if anyone knows the input impedance of the older Quad II tube amps. I'm looking to choose a passive preamp for it from Akustyk. I was advised to choose 50k and that this should be 1/3 of the input impedance. I just want to make sure thats cool before pulling the trigger
Thanks
Ryan
Howdy, I'm wondering if anyone knows the input impedance of the older Quad II tube amps. I'm looking to choose a passive preamp for it from Akustyk. I was advised to choose 50k and that this should be 1/3 of the input impedance. I just want to make sure thats cool before pulling the trigger
Thanks
Ryan
Hi there, I just picked up a nice set of nearly original Quad II's. It looked as though R12 and C5 had been replaced in each at some point. They were each different and I couldn't see the values on the resistors so I thought I would change them each to be the same.
I replaced R12 and C5 with: 696-1068-ND and 399-7533-ND from digikey.
Everything was working well until in one of the amps it got scratchy and then the bass was completely gone. Upon opening it up R12 had come loose. Now I'm no newby with a soldering iron but I thought, maybe it was a cold joint or something. This time I made doubly sure that it was on right; but after another hour of listening the same thing happened. I noticed the tubes were running crazy hot and that there was some new tar leaking from the choke coil. This leads me to think that it is actually melting the solder and the resistor is falling out.
All is just fine with the other amp?
I am running GZ34, Tung-sol KT66's and Tung-sol EF806's
Any ideas what the heck is going on
I replaced R12 and C5 with: 696-1068-ND and 399-7533-ND from digikey.
Everything was working well until in one of the amps it got scratchy and then the bass was completely gone. Upon opening it up R12 had come loose. Now I'm no newby with a soldering iron but I thought, maybe it was a cold joint or something. This time I made doubly sure that it was on right; but after another hour of listening the same thing happened. I noticed the tubes were running crazy hot and that there was some new tar leaking from the choke coil. This leads me to think that it is actually melting the solder and the resistor is falling out.
All is just fine with the other amp?
I am running GZ34, Tung-sol KT66's and Tung-sol EF806's
Any ideas what the heck is going on
Each output tube should be idling at 65mA, so a total of 130mA thru R12.
Measure the voltage across R12. It sould be V=IxR = 0.13 x 180 = 23.4 Volts.and power dissipation in that resistor is just over 3 watts. The Digikey part you specified is a 5 Watt resistor. You should be using at least a 10W resistor for R12.
If you measure more than say 28 V across R12 (no signal) then you have a problem with a KT66 output tube conducting excess current. That would overheat R12, maybe enough to desolder itself and fall out.
This could be due to just a tube being out of spec or a more basic probelm with the most likely of those more basic problems being a leaky C2 or C3 pulling a KT66 grid high in voltage.
Cheers,
Ian
Measure the voltage across R12. It sould be V=IxR = 0.13 x 180 = 23.4 Volts.and power dissipation in that resistor is just over 3 watts. The Digikey part you specified is a 5 Watt resistor. You should be using at least a 10W resistor for R12.
If you measure more than say 28 V across R12 (no signal) then you have a problem with a KT66 output tube conducting excess current. That would overheat R12, maybe enough to desolder itself and fall out.
This could be due to just a tube being out of spec or a more basic probelm with the most likely of those more basic problems being a leaky C2 or C3 pulling a KT66 grid high in voltage.
Cheers,
Ian
Last edited:
Thanks for the replies guys. I will change C2 & C3 and while I'm in there I might as well do C4 & C6. Once that's done I will take some measurements too and post the results.
For C2 & C3 do these look suitable www.thetubestore.com - Mojotone Vitamin T Capacitor .1uF/600V
I have seen a few different opinions on the values of C4 & C6 when using the GZ34 but I could use a bit of guidance to help choose something. The original values are 16+16uf, I have seen kits on ebay that come with 50+50uf and I recall a webpage somewhere that recommended 33+33uf. The 16uf and the 50uf seem available here:
www.thetubestore.com - F&T Can Capacitor 16uF x 16uF / 450V
www.thetubestore.com - F&T Can Capacitor 50uF x 50uF / 500V
Thanks
For C2 & C3 do these look suitable www.thetubestore.com - Mojotone Vitamin T Capacitor .1uF/600V
I have seen a few different opinions on the values of C4 & C6 when using the GZ34 but I could use a bit of guidance to help choose something. The original values are 16+16uf, I have seen kits on ebay that come with 50+50uf and I recall a webpage somewhere that recommended 33+33uf. The 16uf and the 50uf seem available here:
www.thetubestore.com - F&T Can Capacitor 16uF x 16uF / 450V
www.thetubestore.com - F&T Can Capacitor 50uF x 50uF / 500V
Thanks
C2 and C3 are specials. They have a case that is connected to ground and stop the EF86 from taking off. Don't change them or it may become unstable!
As a matter of interest, the over priced Vitamin T capacitors are placed on a copy of the schematic for the 1949 designed Williamson amplifier.
As a matter of interest, the over priced Vitamin T capacitors are placed on a copy of the schematic for the 1949 designed Williamson amplifier.
Last edited:
Your right, I just found some K4oY's and ordered them. I guess the allure of not waiting 2+weeks to get them appealed to me.
After changing these I will measure C4 & C6 as per keith Snook's page as see what to do there.
Possibly just my opinion, but I hate seeing these (and other) classic amps "improved" with boutique bits. I would agree with ColinA regarding boutique components - in general! Anecdotal views regarding such abound - but I will not go into those marshy areas here!
Preserving 'originality' of a classic instrument can be contentious - in some cases one can either have an original model or an operational one, not both. That regarding R12 in this case. Component specs are usually at 25°C, and the under-chassis temperature for Quad II is 35° - 40°. Then there is the normal spread in voltages etc. The spec of 3W was wrong. I myself go to a 10W component if only to distrbute the heat source over a wider source area. And please mount it as far a possible from right under the choke and close to C5! I have yet to find a Quad II where C5 was not baked bone-dry.
While commenting, the capacitance to common of C2, C3 often comes up. It is about 18pF. Not having that showns a small peak in response at about 70 KHz. or an overshoot in square wave response. None of this is serious enough to cause danger of instability, even with capacitive loads. But if academic correctness is important, one can mount such capacitors on the KT66 grids. A better way is to add some phase lead in the NFB circuit by a 1nF capacitor across R11.
Also: When testing for leaky coupling caps C2-C3 by checking the KT66 control grids' d.c. voltage: These are not zero, by effect of returning R7, R8, R9 to the positive bias voltage on the EF86s' cathodes instead of common. This also results in a slightly lower voltage on V2 anode, but the given operating voltages are mostly swamped by manufacturing spread of tubes (particularly so these days).
To answer the original question: The input impedance of a Quad II is 1,5Meg shunted by 10pF.
Preserving 'originality' of a classic instrument can be contentious - in some cases one can either have an original model or an operational one, not both. That regarding R12 in this case. Component specs are usually at 25°C, and the under-chassis temperature for Quad II is 35° - 40°. Then there is the normal spread in voltages etc. The spec of 3W was wrong. I myself go to a 10W component if only to distrbute the heat source over a wider source area. And please mount it as far a possible from right under the choke and close to C5! I have yet to find a Quad II where C5 was not baked bone-dry.
While commenting, the capacitance to common of C2, C3 often comes up. It is about 18pF. Not having that showns a small peak in response at about 70 KHz. or an overshoot in square wave response. None of this is serious enough to cause danger of instability, even with capacitive loads. But if academic correctness is important, one can mount such capacitors on the KT66 grids. A better way is to add some phase lead in the NFB circuit by a 1nF capacitor across R11.
Also: When testing for leaky coupling caps C2-C3 by checking the KT66 control grids' d.c. voltage: These are not zero, by effect of returning R7, R8, R9 to the positive bias voltage on the EF86s' cathodes instead of common. This also results in a slightly lower voltage on V2 anode, but the given operating voltages are mostly swamped by manufacturing spread of tubes (particularly so these days).
To answer the original question: The input impedance of a Quad II is 1,5Meg shunted by 10pF.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.