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Old 3rd November 2012, 09:55 PM   #81
rongon is offline rongon  United States
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Location: Across the river from Rip's big old tree...
Quote:
Originally Posted by SandyG View Post
Here is the second test. This is with the GZ34, R16=4.7K, All tubes in and no signal applied.

R7=2.2K
R8=270K
R9=R14=150k

Voltages
HV1=348
HV2=347
HV3=343
BIAS=23.3
R7=1.45
R8=215
R9=125
R14=126

Clipping at 9.1VRMS into 8ohm load. Possibly a tick better then the previous test. A bit over 10 watts.

The numbers you calculated for the VP and VK of the input look to be about what you have in your last calculation. Numbers seem to work, might be the best that can be had with the design. I'm going to drill out a test chassis to mount all the mess up the plug into a speaker to see what it sounds like.

Sandy
I have a question. When you report the voltage...

R8=215

...does that mean there are 215V dropped across R8? If so, is the voltage at the plate of the first stage 6SL7 = +128V?

Thanks.
--

I looked at the GE datatsheet for 6V6GTA, and for a single 6V6 in class A, with a plate voltage of +315V and a screen voltage of +225V, you're supposed to get 5.5W output at 12% THD. I don't think it's unreasonable to expect only 10W from a push-pull pair of 6V6GT in class A, especially if used ultralinear (screen taps on the OPT).

Last edited by rongon; 3rd November 2012 at 10:00 PM.
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Old 3rd November 2012, 10:38 PM   #82
rongon is offline rongon  United States
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Quote:
Numbers seem to work, might be the best that can be had with the design.
How do you know that for sure? That's what I'm trying to figure out.

Can you tell us what is the voltage at the first stage 6SL7 plate?

Can you tell us what is the voltage at the second stage 6SL7 cathode?

Just those two voltages, nothing else, please. Those are two very important voltages.

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Old 4th November 2012, 05:39 PM   #83
SandyG is offline SandyG  United States
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Rongon -

Made a couple of posts yesterday night to reply, but not sure why they didn't show and oddly got notified again of your last post. Not sure what happend, but I'll try to remember what I wrote...

First off, I forgot to include the plate voltage in the early measurements. I'll repost the last 2 results with those numbers.

To be clear on what the numbers are HV1,HV2, HV2, Bias, V1PA, V1PB are measured at the check points on the schematic. The important ones that I forgot to included were V1PA (Input Plate), V1PB (Phase Inverter Plate), I had them measured, but for some reason skipped putting them down in the post!

The rest of the resistors are measured across the resistor showing the drop.

R14 will show the phase inverters Cathode voltage as this resistor is from cathode to ground in this case.

TEST 1
R7=1.2K
R8=220K
R9=R14=150k

Voltages
HV1=346
HV2=345
HV3=339
BIAS=23.1
V1PA=112
V1PB=224
R7=1.1
R8=224
R9=113.7
R14=114.9


TEST 2
R7=2.2K
R8=270K
R9=R14=150k

Voltages
HV1=348
HV2=347
HV3=343
BIAS=23.3
V1PA=124
V1PB=217
R7=1.45
R8=215
R9=125
R14=126

Let's hope I copy&pasted better this time...

Sandy
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Old 4th November 2012, 05:50 PM   #84
SandyG is offline SandyG  United States
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Here is what the chassis is looking like...
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File Type: jpg IMAG1570.jpg (720.6 KB, 181 views)
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Old 4th November 2012, 11:27 PM   #85
rongon is offline rongon  United States
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Yes, that's what I wanted to know.

If V1PA = 124V, and the cathode of V2 (R14) is 126V, then there is -2V grid bias on V2.

If there are 126V dropped across R14, then V2 is drawing 0.84mA. That's not bad.

126V/150,000R = 0.00084A

It all makes sense now.

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Old 6th November 2012, 04:01 PM   #86
SandyG is offline SandyG  United States
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A shot of the EZ10 testing. With only one amp hard to give it a thorough testing, but I swapped out one of the EZ260 Grommes amps with the 10watt version and it sounds very clean and quiet. Less punch, but that's to be expected.

I have since taken it to my office and where I have one of those Chinese SET KT88 headphone amps (Noisy, hummy, cheep) and swapped out one channel and it sound very good, possibly a bit more punch. This is running a smaller speaker (polk audio) and smaller room where I intend to use it so just need to get the second set of transformers from EDCOR and build the second channel.

I also swapped out the GZ34 with a 5U4GB to see if I could hear anything but seemed similar, but hard to tell with only one channel. Will be trying this test again just to see if I can hear a difference.

I think the mini-monoblocks will work out nice for my computer audio system.

One thing to note is the gain is bit lower then the other amps I have, but doesn't seem to be a problem with what I'm driving it with. Some of the changes in the design may also have reduced gain a bit, but really don't think it makes a huge difference when all said and done.

Again want to thank everyone that chimed in with suggestions and help!!!!

Will post the completed amps once I get the transformers and chassis painted all up.

Sandy
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Old 6th November 2012, 05:18 PM   #87
rongon is offline rongon  United States
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Sounds good!

Re: Lower gain -- Vintage amps had too much gain for today's sources. A computer sound card can usually send 1Vrms signal. CD players are supposed to send 2Vrms (2.83V peak) audio signal output. So the amp should be able to accept 1.5V to 2V peak signal at its input without clipping (leaves a little headroom for weak sources). Most vintage amps would clip with as little as 0.5V peak signal at the input.

Also, if you're using a log-taper, carbon-track potentiometer as your volume control, it will usually sound better if that control is turned up to 10 o'clock to 2 o'clock or so for normal listening, instead of down at around 8 o'clock.

As long as the amp reaches full output with the vol control turned up nearly all the way, then you're fine. I see no point in having a system where turning it up half-way causes clipping.

Is that Stereo 70 working? It looks like it's been modded. How does your EZ10 sound in comparison to that?

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Old 7th November 2012, 12:24 AM   #88
SandyG is offline SandyG  United States
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Quote:
Originally Posted by rongon View Post
Sounds good!

Re: Lower gain -- Vintage amps had too much gain for today's sources. A computer sound card can usually send 1Vrms signal. CD players are supposed to send 2Vrms (2.83V peak) audio signal output. So the amp should be able to accept 1.5V to 2V peak signal at its input without clipping (leaves a little headroom for weak sources). Most vintage amps would clip with as little as 0.5V peak signal at the input.

Also, if you're using a log-taper, carbon-track potentiometer as your volume control, it will usually sound better if that control is turned up to 10 o'clock to 2 o'clock or so for normal listening, instead of down at around 8 o'clock.

As long as the amp reaches full output with the vol control turned up nearly all the way, then you're fine. I see no point in having a system where turning it up half-way causes clipping.

Is that Stereo 70 working? It looks like it's been modded. How does your EZ10 sound in comparison to that?

--
The Stereo 70 is not an original It's a Bob Latino ST70 with a few of his upgrades. It's a very clean sounding and I want to say FAST and sensitive amplifier if that has any meaning in how to describe the sound. I also have a pair of modified Dynaco MKIII with the Triode Electronics Driver boards that I restored. And my favorite sounding is the Grommes 260A Clones (EZ260). They had the worst waveforms, but sound the best to me. I would rate them in how I enjoy the way they sound as EZ260, ST70, MKIII's. The ST70 that Bob Latino does is a great amp, very powerful, and it's really what got me going on the tube thing. I have to work on a pre-amp, the stock PAS-2 that I have is very tired. I have one of his PAS-2 upgrades, just have not got to finishing it it yet.

The funny thing was the EZ10 was going to be used as a simple amplifier for some of my transistorized HAM gear to get some of the old tube sound that I enjoyed, who knew it would sound so good

I think I have one more power amp in me, the EZ10 was a better project to learn some things (I think I could do a load line now!). Will be getting back to the EZ125 wth 6SN7's and 4 Kt88 as time permits...

Sandy
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Old 7th November 2012, 01:38 AM   #89
rongon is offline rongon  United States
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Re: Dyna PAS2 - If it's even close to stock, then you can do much better, and pretty easily too.

Do you spin vinyl, or are you all-digital for your source material? The only signal source that really needs its own dedicated preamp is a record player.

If you're all-digital, then you don't even need a line stage with gain. The lowest distortion preamp is no preamp at all. All the amps you have can likely be driven to clipping with the 2.83V peak out from a CD player (your EZ10 can). If your interconnect cables are no longer than about 6 feet or so, you might consider putting together a box with the necessary input and output jacks, a high quality input selector switch and a high quality stepped attenuator. You could even use an "inductive attenuator" like those from Intact Audio.

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Old 7th November 2012, 05:38 PM   #90
SandyG is offline SandyG  United States
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Source is mostly digital, but call me crazy I like tone controls. I'm not a fancy purist or anything like that, so for me I'm ok with the added 'color' a preamp has. I have a couple of un-built Glassware Akido preamp boards and that is also in the works, but I have a nice Dynaco PAS-2 project with upgraded preamp board (Tubes4Hifi) BUT retaining the tone controls.

I think of it this way, it's my music I'll make it sound the way I like to listen to it and if I have a bad mix I can turn a knob and make it enjoyable even if not accurate

Sandy
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