DIY Audio Projects Forum

Hi Matt, Did you ever retrofit your amp with the GXSE transformers? I'd be curious to see what your impressions were...

Not yet. I have two versions of the upgraded amp on the drawing board. One is a smaller amp with the same power transformer, the other is a pair of mono-blocks with tube rectification. I haven't gotten beyond the basic design stage with either yet. Maybe some time this summer.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
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Hi Guys. I am new to this forum and tubes. Great info here Thanks. About to build this amp but have the Hammond 369EX transformer. This PT has a center tap on the 6.3v secondary. Should I remove the 100ohm resistors and connect this to earth or a cathode or just ignore it.
Thanks
Steven

Just omit the two 100Ω resistors on the filament lines and tie the center tap from the 6.3v winding directly to signal ground.

The two 100Ω resistors form a virtual ground for the filament circuit if the transformer does not have a center tapped filament winding. I actually prefer using center tapped filament windings if they are available.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Thanks Matt. Much appreciated. I will do that.

Steven

I have built up this amp as per the schematic except with a SS power supply. I use it primarily for my 32 ohm headphones in parallel with a 32 ohm resistor - so 16 ohm output load.

I quite like the warmth of the second harmonic distortion, but at times I find the distortion too high for my taste. I am wondering if there is something I can do to bring down the distortion a little without dramatically effecting the sound quality. I have toyed with a small amount of global negative feedback or perhaps changing the operating points of the tube?

Any advice would be appreciated.

Just finished up this circuit in a wine box we had sitting around. Since Suncalc likes to name his amps after wood, this one will be called "Spottswoode" .
I'm not sure whether or not I have an issue though. It seems to have fairly low outputs. When driven by the iPhone (set at max volume), I don't get any output until the volume knob is around the 11 o'clock position. Maxed out it finally starts pushing the speakers (85.5db sensitivity). It sounds really, really good, but it seems like it should have more to it. Or maybe my ears are just trashed?

Measured average voltages of both 6cy7's:
Plate Section 1 - 127v
Plate to Cathode Section 1 - 125v
Plate Section 2 - 222v
Plate to Cathode Section 2 - 195v

I've gone back over everything a few times now, and can't seem to find anything obvious. (I am a hack, though...)
Swapped in a different set of tubes with no change (although I should test them).
If anything stands out in the pictures, or the voltages look wacky - let me know.
Maybe I am just expecting too much out of the little fella. Or maybe I am really hard of hearing.

Anyhow, thanks to Suncalc for sharing this design! Even if this is all the output it has, it will still get put to good use.

First, the amp looks great. But you are right, your sensitivity seems low.

By design, the unbypassed driver stage has an amplification of 24.7v/v (≈27.8dB). So with a signal of 1v peak in it should be fully driving the power stage to about a watt of output. Your iPhone should be producing between 1.5v and 2.0v peak. Your plate voltages are in the right range. Since this is happening in both channels, it is likely something systemic rather than a simple bad solder joint or missed wire.

First I would check resistor values. Sometimes a decimal can get off set causing this type of behavior. Second, make sure that they're aren't any subtile wiring errors. Go through the amp carefully and match each connection within the amp with a path on the schematic. Make sure that you didn't make a mistake in the translation that slipped into both channels. This can sometimes happen if you use the first channel wired as a physical model for the second.

I will also say, swap speakers with something a little more sensitive; like a small bookshelf speaker in the 90dB range. Sometimes low sensitivity speakers with complicated crossovers can take substantial drive. I don't know what speakers you're using but the key here is to swap a few things and see what changes. You already did tubes, so you know its not that.

Let us know what you find.

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Thank you Matt. I have gone back over things repeatedly between last night and this morning.
I verified all resistor values. Everything checked out perfectly, with the exception of the 100ohm Dale resistors for the heaters. They were reading around 28Meg for some odd reason. They're clearly labeled 100ohm, so maybe I fried them while soldering. Anyhow, replaced those, and then tested all of 6cy7's I had tried in the amp so far. All tested good.

I have never used an Alps pot before, and one thing I noticed is that it still has around 90k resistance between input and wiper when the knob is at 12 o'clock. Does that seem excessive? Also, does the pot body need to be isolated from the chassis?

Pardon the dumb question, but should I have 100k resistance between the center pin of the rca input and ground at all times?

Also, speakers are 4" full range Mark Audio's. I'll give a more sensitive pair a try.

Yes. The following is what the input wiring should look like:As you can see, regardless of the setting on the control, the entire resistance of the control shows up between the RCA input and ground. This is why the value of that control largely sets the input impedance of the amplifier.

No. Actually that is normal. In order to have a linear effect on perceived hearing, the taper function of the pot needs to be logarithmic. This is because the human ear is a logarithmic detector. The figure below shows the "theoretical" transfer function for a 100kΩ "A" function audio taper control.This plot shows the taper resistance to "ground" of an audio volume control by shaft position. As you can see, at 50% shaft rotation (i.e. "12 o'Clock) the resistance is only 10kΩ to ground. This leaves 90kΩ between the input and the wiper as you saw. This is exactly how the control should function.

This is a good idea. This amp, while it has a great sound, is no powerhouse by any means. I remember having a set of vintage audiophile Audio Research AR-4x speakers from the late 1960s in my shop for refurbishment. As a test, I connected the vertical amp to them for a function check and I was shocked at how little volume they produced. I don't know what the actual sensitivity was, but I know that AudioResearch suggested in their advertising copy a minimum of 15W per channel amplifier. By comparison with other speakers I had in the shop at the time, I estimated their sensitivity in the low to mid 80dB range.

It was only after thinking about it for a while that I realized that a simple 3dB difference in speaker sensitivity meant the spearers only sound half as loud. By the time you get to a 6dB difference it's really shocking how little sound comes out of an insensitive speaker. This is one of the reasons that today, most people recommend simple high sensitivity drivers (mid 90dB range) for these small SETs. I routinely use small 2-way bookshelf speakers with sensitivity of about 90dB with this amp for casual listening.

I hope this helps.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog