A question about channel separation with ECC81 and 1625 tubes

[Leoncino]

Dear all,

this is my first post on this forum (actually the second, considering my introduction in the dedicated section).

Several years ago I built an single ended tube amplifer based on 1x ECC81 and 2x 1625: the amplifier was working nicely, but then, for several reasons which are not really important to explain now, it stayed closed in a wooden box for, at least 15 years. Recently I have decided that it is really a pity to keep all the compontents in a dummy box and therefore I have decided to finally finalize this project, building a suitable chassis. In the following picture you can find the schematics which I was using (probably you already saw that several times and I shall credit RH design, who designed it).
In my case I am using the 1625 tubes, by taking in consideration the differences between this tube and the 807. I am using only one ECC81, as prescribed in the schematics.

Some days ago, while I was asking about the possibility to implement an audio taper (question which is however nor completely closed), I got a reply by an user who said that I shall use a pair of ECC81 instead of only one. This is, according to that user, because of channel separation. He assumes that with the configuration below I will not have channel separation.

At first glance I was already calculating if my power transformer (which has been built tailored to the schematics) has enough margin to provide enough current to another ECC81 (heater and anodic), but then I started to get suspicious about the real need to implement another tube. Of course I have a transformer which has margin, but if I will need to implement another tube, it will of course be reduced.

I would therefore ask your opinion on this topic, because I am not totally convinced about the "loss of stereo effect" which could derive by using only one tube on the first stage.

Thank you very much for your patience,

Regards,

Marco

 

[gadget73]

I'm assuming the notion was use 1/2 of a tube for each channel? Honestly the amount of signal bleed from channel to channel caused by using a single tube is insignificant. More likely to get bleed from bad wire routing. With some measurement gear you can quantify it easily enough but I wouldn't bother re-inventing the wheel here. Tons of amplifiers shared tubes like this and channel separation isn't a concern.

as for heater capacity, if you really want to do this, you don't actually need more power transformer. The ECC81 has two heaters in it. Normally you tie 4 and 5 together to one side of the line and 9 to the other side for 6v operation. You can power just pin 4 and 9 or 5 and 9 to light half the tube. 150ma of heater load vs 300ma at that point.

If you want to give yourself a "free tube", use opposite halves of the tube. Pins 1-3, heater on pin 4 for one channel and 6-8 with heater on 5 for the other. That way one triode will be brand new on each side, and when you eventually wear them out you can simply flip the tubes and get two fresh triodes to use.

 

[ramjet]

Dear all,

this is my first post on this forum (actually the second, considering my introduction in the dedicated section).

Several years ago I built an single ended tube amplifer based on 1x ECC81 and 2x 1625: the amplifier was working nicely, but then, for several reasons which are not really important to explain now, it stayed closed in a wooden box for, at least 15 years. Recently I have decided that it is really a pity to keep all the compontents in a dummy box and therefore I have decided to finally finalize this project, building a suitable chassis. In the following picture you can find the schematics which I was using (probably you already saw that several times and I shall credit RH design, who designed it).
In my case I am using the 1625 tubes, by taking in consideration the differences between this tube and the 807. I am using only one ECC81, as prescribed in the schematics.

Some days ago, while I was asking about the possibility to implement an audio taper (question which is however nor completely closed), I got a reply by an user who said that I shall use a pair of ECC81 instead of only one. This is, according to that user, because of channel separation. He assumes that with the configuration below I will not have channel separation.

At first glance I was already calculating if my power transformer (which has been built tailored to the schematics) has enough margin to provide enough current to another ECC81 (heater and anodic), but then I started to get suspicious about the real need to implement another tube. Of course I have a transformer which has margin, but if I will need to implement another tube, it will of course be reduced.

I would therefore ask your opinion on this topic, because I am not totally convinced about the "loss of stereo effect" which could derive by using only one tube on the first stage.

Thank you very much for your patience,

Regards,

Marco

View attachment 3586460

One 12AX7 shared between right and left channels on McIntosh MC225, Mc240, Mc275, MA230, and MAC1500, just to name a few. I own a few of these and channel separation (lack of), has never been a problem.

 

[Leoncino]

I'm assuming the notion was use 1/2 of a tube for each channel? Honestly the amount of signal bleed from channel to channel caused by using a single tube is insignificant. More likely to get bleed from bad wire routing. With some measurement gear you can quantify it easily enough but I wouldn't bother re-inventing the wheel here. Tons of amplifiers shared tubes like this and channel separation isn't a concern.

as for heater capacity, if you really want to do this, you don't actually need more power transformer. The ECC81 has two heaters in it. Normally you tie 4 and 5 together to one side of the line and 9 to the other side for 6v operation. You can power just pin 4 and 9 or 5 and 9 to light half the tube. 150ma of heater load vs 300ma at that point.

If you want to give yourself a "free tube", use opposite halves of the tube. Pins 1-3, heater on pin 4 for one channel and 6-8 with heater on 5 for the other. That way one triode will be brand new on each side, and when you eventually wear them out you can simply flip the tubes and get two fresh triodes to use.

Thank you very much for your reply, I will go ahead with only one triode. However, your observation about the heaters of the ECC81 is very interesting!

 

[Leoncino]

One 12AX7 shared between right and left channels on McIntosh MC225, Mc240, Mc275, MA230, and MAC1500, just to name a few. I own a few of these and channel separation (lack of), has never been a problem.

This is very good to know!

 

[Leoncino]

I don´t want to profit of your patience... do you think that the following modification to the input stage could be worth and correct to regulate the volume? Or did I do some mistakes? Thank you

 

[kward]

Channel separation is absolutely not an issue with that design. Also the screen resistor in the output stage at 10k is too large. Reduce it to something in the range of 100 ohms to 1k ohms.

 

[kward]

I don´t want to profit of your patience... do you think that the following modification to the input stage could be worth and correct to regulate the volume

What are you driving the amp with? You can very likely change the volume pot to 100k and replace that 0.22uF cap with a 10 k resistor.

 

[kward]

Also bring the 1M resistor to the left and attach it directly to the wiper of the volume pot.

 

[Leoncino]

What are you driving the amp with? You can very likely change the volume pot to 100k and replace that 0.22uF cap with a 10 k resistor.

I will drive the amp with a CD player and eventually with a record player (which has its one integrated RIAA amplifier).

 

[kward]

A CD player (and I believe an integrated phono preamp) will have low output impedance on the order of a few hundred ohms as well as have no DC on the input line, so you can do away with the coupling capacitor, and you should be able to use any size volume pot you want from say 25k to 100k.

 

[Leoncino]

Thank you very much!!!

 

[Leoncino]

as for heater capacity, if you really want to do this, you don't actually need more power transformer. The ECC81 has two heaters in it. Normally you tie 4 and 5 together to one side of the line and 9 to the other side for 6v operation. You can power just pin 4 and 9 or 5 and 9 to light half the tube. 150ma of heater load vs 300ma at that point.

If you want to give yourself a "free tube", use opposite halves of the tube. Pins 1-3, heater on pin 4 for one channel and 6-8 with heater on 5 for the other. That way one triode will be brand new on each side, and when you eventually wear them out you can simply flip the tubes and get two fresh triodes to use.

Hi, I was thinking again to your reply and I was wondering which could the way to do that with 12.6 Volt (my transformer has only two secondary, one for the HV and one which provides 12.6V).

 

[gadget73]

Can't do it with a 12.6v supply unfortunately. It only works when used in 6v mode. I forgot the 1625 is not a 6v tube.

 

[gadget73]

hm, you know what would work though, a pair of 6AB4. Thats 1/2 a 12AT7. Wire the heaters in series. At that point it would just be a 12AT7 spread across two sockets. Its a 7 pin tube though, which might be a concern if you already have a hole punched for a 9 pin. I have seen 7 pin sockets sized that will fit into the cutout for a 9 pin but that might be one of those deals where you have to find oddball old production stuff.

 

[kward]

Are these monoblocks? If yes and you want to continue to use one-half of a 12AT7 (ECC81) per monoblock, I would just wire up the filament between pins 4 and 5 (thus for 12.6V operation), then wire up the first triode as the voltage amp as shown above. Then, trickle a few milliamps of quiescent current through the second triode (since it's filament is lit, you need to do something to prevent cathode poisoning over time). Wire the triodes in opposite configuration for the other monoblock (the second triode as the voltage amp, the first triode trickle a few mA through it). You can then swap 12AT7 tubes between monoblocks from time to time to equally consume both triodes and wear the tubes evenly.

Otherwise if this is a two channel amp built on one chassis with one common power supply then there's no issue--use the 12AT7 first and second triodes for the left and right channel respectively, and done.

Or am I missing what you are trying to accomplish?

 

[gadget73]

Fair point, I had assumed it was stereo since OP mentioned 2x 1625 tubes.