[ruZZ.il]

BOM

 

[tomb]

Quote:

Originally Posted by CaptHowie Hey tomb, I'm having trouble understanding the new schematic. I know that schematic is just for the mono channel, but could you please create a stereo schematic like the original with all the labels? I'm a schematic noob so i'm just having a bit of trouble Thanks for all the help you've provided to the SS and i'm looking forward to the PCB's as i'm a bit nervous about point-to-point :|

Sorry, no. Cut and paste to have two schematics, then draw a line from the "+" connection on one to the "+" connection on the other. Dsavitsk did the schematic anyway.

Quote:

Originally Posted by ludoo /img/forum/go_quote.gif Tomb, are 16V for C7/C8 really needed? I was thinking of getting some 6.3V os-cons as cathode bypass caps...

It's quite possible that 6.3V os-cons are enough, but maybe not. It think we only have ~4V in the MAXes around the cathode bypass, but the SSMH has a much higher voltage threshold.

Since the cathode bypass caps are optional, I'd measure the voltage at the pads after you get it built and powered up. It would be easy to install the two caps after that.

Quote:

Originally Posted by MrMajestic2 I missed ordering the bypass caps. How important are they and how important is it that they are bipolar?

I think it's very important that they're bipolar - they meet the primary definition of a bipolar cap: voltage swinging in both directions at all times. However, a regular cap has performed just as well (supposedly) in other amps. In any event, they're optional.

Quote:

Originally Posted by MASantos Tomb, which potentiometer did you choose for this? I can't recognise the footprint in the board.

As ruZZ.il says, check the BOM.

We are using the same Alpha pot that Pete originally spec'd.

 

[ludoo]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Since the cathode bypass caps are optional, I'd measure the voltage at the pads after you get it built and powered up. It would be easy to install the two caps after that.

Good idea. I could also have a look at the 19j6 curves and try to understand what's the cathode/grid voltage.

Edit: as for them being bipolar, one side (-) is connected to ground and the other side (+) to the cathode which should be positive with respect to ground, why do you say they would see voltage swinging both ways?

 

[tomb]

Quote:

Originally Posted by ludoo /img/forum/go_quote.gif Good idea. I could also have a look at the 19j6 curves and try to understand what's the cathode/grid voltage. Edit: as for them being bipolar, one side (-) is connected to ground and the other side (+) to the cathode which should be positive with respect to ground, why do you say they would see voltage swinging both ways?

Dsavitsk will have to give you a better explanation than I can. Suffice to say the signal wave amplitude varies up and down. The bypass cap allows a little extra "room" for the wave to expand on the back side of the tube - both negative and positive portions of the wave. If the cathode bypass cap is not there, the actual bias on the tube alters in response to the wave. This is because the tube reacts in reverse to a negative vs. postive signal. This means the cathode bypass cap increases gain - particularly on the lower frequencies, which often have higher amplitudes. This assumes that the cathode bypass cap is selected for a sufficient size that the RC filter formed by the cap and the resistor do not filter out the bass frequencies.

In any event, as the music signal passes through the tube, the capacitor is asked to charge and discharge in both directions, keeping the bias difference of the tube at the same value.

I think that's right, but let's see how much Dsavitsk corrects me.

 

[MrMajestic2]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif I think it's very important that they're bipolar - they meet the primary definition of a bipolar cap: voltage swinging in both directions at all times. However, a regular cap has performed just as well (supposedly) in other amps. In any event, they're optional.

Ah, I see. Hmm, does anyone have a couple of bipolars like in the BOM to sell me

 

[dsavitsk]

Quote:

Originally Posted by ludoo /img/forum/go_quote.gif Edit: as for them being bipolar, one side (-) is connected to ground and the other side (+) to the cathode which should be positive with respect to ground, why do you say they would see voltage swinging both ways?

Based on the tube curves (I don't have a working SS in front of me to measure), it looks like the cathodes are biased up about 1 to 1.5V. Under normal circumstances, a polar cap should be fine. As Tom notes, there is signal swing, but it will typically never drive the voltage fully the other way.

However, that we do usually think of 2V signals in audio. A 3V peak to peak signal will drive the grid just positive which will reverse the voltage on the cap. Now, this would overdrive the amp well past the point of unpleasantness so you wouldn't be listening to it there. Also, most electrolytics are OK with a small reverse voltage for a short period, so again, you are OK. However, OS-CONS may be the exception to that as they are a little more sensitive to voltage than other electrolytics.

All said, I think polar caps are safe to use, and in fact I think 6.3V is probably plenty. But, do be aware of voltages, and take some measurements to be sure. In the end, if the cap breaks, the worst thing that happens is it leaks a little, and if it shorts completely the amp will lose bias and distort a lot and you'll know what to fix. It won't damage phones, or the other components, or anything like that -- there won't be any tube runaway.

Also, only use this cap if you need more gain. Otherwise, leave it out.

 

[ludoo]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif Based on the tube curves (I don't have a working SS in front of me to measure), it looks like the cathodes are biased up about 1 to 1.5V. Under normal circumstances, a polar cap should be fine. As Tom notes, there is signal swing, but it will typically never drive the voltage fully the other way. <snip...>

Thanks for the really excellent explanation.

I was at the office today and had no meter, but I can open up my home SS and measure the grid/cathode voltage if it's of any interest.

I will probably put in cathode bypass caps as orthos are very hard to drive, and also I'm curious about any sonic differences (if any) with my previous SS that have no cathode bypass caps.

Edit: I just slightly raised one tube and measured the voltage between cathode and grid, and I get 0.92V.

 

[dsavitsk]

Quote:

Originally Posted by ludoo /img/forum/go_quote.gif Edit: I just slightly raised one tube and measured the voltage between cathode and grid, and I get 0.92V.

That's about what I figured. With the cathode bypass in place, we may be able to lower the load on the tube (and lower Rk) and increase current through it keeping bias in about the same spot. By doing this, rp will be lowered which might make the proportional load actually higher. Mu, incidentally, will be raised both due to the cap eliminating the degenerative feedback, but also because it increases with current. It will take some experimentation to figure out where these tubes like to run, and for all I know, Pete already did this and settled on where it is. However, the datasheet (look at the bottom graph on the last page) seems to indicate that increasing the current through the tube will be a good thing. So, I think the experiment is worthwhile.

 

[iareConfusE]

Any suggestions for the volume knob? I didn't see it on the BOM and I'm not sure what the dimensions should be. I looked at the manufacturer's page for the potentiometer, and couldn't find any knobs that matched the shaft's dimensions.

Also, will pre drilled face plates for the Hammond case be sold or are we going to have to drill them ourselves?

 

[scompton]

Quote:

Originally Posted by dsavitsk /img/forum/go_quote.gif Also, only use this cap if you need more gain. Otherwise, leave it out.

So if I want less gain, I should skip this cap and add 50K resistors between the input and the pot?

 

[dsavitsk]

Quote:

Originally Posted by scompton /img/forum/go_quote.gif So if I want less gain, I should skip this cap and add 50K resistors between the input and the pot?

Yes

 

[zkool448]

Quote:

Originally Posted by CaptHowie /img/forum/go_quote.gif I know that schematic is just for the mono channel, but could you please create a stereo schematic like the original with all the labels? I'm a schematic noob so i'm just having a bit of trouble

CaptHowie, if I add these optional caps/resistors this is exactly how I would connect/add them to Pete's original schematic... *however* before I do I'll need someone here first to kindly verify my diagram! thanks

zk

 

[tomb]

Quote:

Originally Posted by zkool448 /img/forum/go_quote.gif CaptHowie, if I add these optional caps/resistors this is exactly how I would connect/add them to Pete's original schematic... *however* before I do I'll need someone here first to kindly verify my diagram! thanks zk

Looks fine to me! Very good! How about I post that one on the website, too?

 

[zkool448]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif Looks fine to me! Very good! How about I post that one on the website, too?

Thanks TomB.

By all means, please post it on the website if you think it helps.

zk

 

[nightanole]

I have a question on the bypass caps. I guess i miss read the Bom of one of the early builds. Anyway i have 2 sets of .22uf audio caps coming, the Wima MKP10 .22uF 250V from mouser and the vitamin C's from beezer. I was planning on using the wima as by passcaps on the power supply, and the vitamins on the bypass caps on the outputs. Now i realize i still need another set of .22 coming off the tubes. So which set should i use for c2,c4 and which set for c3a/c5a since i dont have enough caps for the power supply bypass ( and its not really needed anyway)?