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Old 3rd January 2018, 12:14 AM   #11
DrAwesome is offline DrAwesome  United States
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Quote:
Originally Posted by Printer2 View Post
A couple of resistors for the heater supply tied to the power tube cathode? I am guessing the output transformer will be the weak link in the system. I doubt they put more transformer than was needed.
Not quite sure where these resisters you speak of reside? But the transformer actually powered several more tubes so I think it may be a strong link
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Old 3rd January 2018, 07:56 AM   #12
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by DrAwesome View Post
.
And I eliminated the “hum balance. I’m not sure what that was doing- I took it out.
Do yourself a favor and leave it in. This keeps the filaments to a defined ground level, their windings should cancel hum pickup from the cathode then.
You have read it - it's only two resistors.

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Originally Posted by DrAwesome View Post
The other question I have is for the 5000micromicrofarad (oops- no value labeled!but that’s what those are) capacitor‘s in the power supply by the 6X4- I don’t see those with other rectifying circuits. Anyone know about that?
5000µµF Long time I since I saw such a term. The engineers of that time obviously loved the power of ten. I wonder why they did not call it 5kpF
These are suppressor caps, actually not uncommon in rectifier circuits. I think you can omit then. Stick to the heater balance resistors instead.

Quote:
But the transformer actually powered several more tubes so I think it may be a strong link
Could it be you both are talking about different transformers?
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Old 3rd January 2018, 02:37 PM   #13
DrAwesome is offline DrAwesome  United States
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Printer2 - I must apologize you clearly said “output transformer” and after further consideration I think I get the resisters comments

So this is what we learned so far... I can’t read, I am impulsive, and my lack of humility may result in a round of diy electro shock therapy. Perfect
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Old 4th January 2018, 03:23 AM   #14
PRR is offline PRR  United States
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Originally Posted by DrAwesome View Post
...6CU5 states that the maximum plate voltage is 135volts maximum screen voltage 117V seems it would be wise to stay within those parameters. ...
Don't be chicken. 6CU5 is *probably* built on the 150V plate-stuff in the 25L6 tradition. Also 6CU5 is about the CHEAPEST tube you can blow-up; I found a price of $5 and I suspect if you hunt a while you can get a 10-pack for $25. It was made in small but not tiny quantity, and apparently didn't sell a lot on the re-tube market. (Who re-tubed their NuTone for better sound on the intercom?)

Also the 6Y6 and friends can work the same, with possible socket and bias changes. But that shoots the price way up: $7.

Yes, there's no rational point in hot-rodding a 1.3 Watt intercom to 1.5 Watts. Still won't fill the stadium; there's other amps for that. 1.3W may be "turn that down!!" in an apartment, and 1.5W won't be enough to drown-out the complaints. So I agree that staying inside Book Limits makes sense. It just isn't Rock'n'Roll.
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Old 4th January 2018, 03:26 PM   #15
Tubelab_com is offline Tubelab_com  United States
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Quote:
no rational point in hot-rodding a 1.3 Watt intercom to 1.5 Watts.
No, the few extra milliwatts afforded by some extra B+ will not make much difference in loudness, but the extra voltage headroom to allow for some downstream sag to the preamp stages will change the entire character of the amp.

Quote:
6CU5 is *probably* built on the 150V plate-stuff in the 25L6 tradition.
The 6CU5 IS a 6 volt 50B5. The first version of the little screamer I built for the HBAC amp challenge used a pair of 50B5's, a 12AV6 and a 12AU6 series wired operating from an isolation transformer. I experimented with B+ voltage from 120 volts to well over 200 volts to find the sound that I wanted. There were no issues operating 50B5's well over 200 volts.

Since the easiest and cheapest B+ voltage choices derived from rectified isolation transformer are 170 volts (full wave bridge) or 340 volts (voltage doubler), I needed to use one of those voltages to minimize cost. The 50B5's will actually live on 340 volts, but the idle current must be in the 10 mA range to avoid over dissipating the tubes, and that just didn't sound good (ugly crossover distortion). I later switched to 32ET5, 18FY6, and 18FW6 tubes because they draw less heater current and are on the dollar menu so the whole amp used $4 worth of tubes.

The schematic is here. This little amp can go from mild and clean to full metal racket just by rolling into the volume knob on the guitar. It is currently the only working guitar amp that I have, and sees use several times a week. It still has the original output tubes in it and the B+ is 170 volts. Screen grid and preamp voltage is about 125 volts until you hammer it hard and then It will drop below 100 volts.

Note....the 18FW6 is an 18 volt 6AU6 which is very similar to the old 6SJ7 pentode used in the Fender 5C1 Champ. The mosfet / pot on the plate allows for a variable plate load allowing the input stage gain to be adjustable from a little, to about 50 dB. The 18FY6 is half a 12AX7. The second mosfet is the phase inverter for true push pull operation. At the time of the original challenge the entire amp could be built for under $50 in parts.

Quote:
Also 6CU5 is about the CHEAPEST tube you can blow-up; I found a price of $5
I can blow up cheaper tubes, but to get tubes for less than $1 you have to buy a bunch! The 6CU5 is on the dollar menu at ESRC. $10 minimum order though.

Summer Dollar Days - Vacuum Tube Sale - $1.00 Vacuum Tubes

Quote:
the 6Y6 and friends can work the same
6Y6's are a bit rare and sometimes pricey. The 12L6 and family (6W6, 12L6, 25L6, 50L6, 12W6, 25W6, and 6GC5) all have the same guts and all but the 6GC5 are octal. The 6L6 and 35L6 are different tubes. The 6GC5 is a fat 9 pin miniature tube. Regardless of what the book says they will all work fine with 300 volts on the plate. I have cranked 40 watts out of a pair on 400 volts, but that IS pushing it a bit. As with some of the small tubes, the plate voltage can be bent a bit (OK a lot) but you MUST respect the screen grid ratings, or meltdown WILL occur.
Attached Images
File Type: jpg Chassis_1_x.jpg (601.0 KB, 39 views)
File Type: jpg Chassis_2_x.jpg (544.0 KB, 33 views)
Attached Files
File Type: pdf GTA_4T_11-04.pdf (22.8 KB, 7 views)
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Old 20th January 2018, 05:47 PM   #16
DrAwesome is offline DrAwesome  United States
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Thanks for the feedback- I'm going to start slapping things together and see what happens. I am humbled by the Tubelab example- beautiful amps. My work schedule has been excessive lately and I haven't been able to accomplish much- Hopefully some progress pics soon
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Old 20th January 2018, 08:52 PM   #17
Tubelab_com is offline Tubelab_com  United States
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Quote:
I am humbled by the Tubelab example- beautiful amps.
It looks pretty now, and I have found the Tolex covered box that it went in, but I haven't married the two yet.

It started out as a piece of perf board with parts sky wired all over it and an entry in the Hundred Buck Amp Challenge. Picture #1.

Then it lost a tube, but gained a real PC board and the Tolex covered box(which didn't fit exactly right). Picture #2 the board is sitting on the speaker cabinet I made for it (given away). Pic #3 shows the board, Pic #4 shows the amp in the box.

About this time, I lost my job of 41 years, and had to pack up all the stuff in the house I had been in for 37 years and move it 1200 miles on 3 weeks notice. That's when I gave away the speaker cabinet and a lot of other stuff. Three years and two moves later I find this little guy stuffed in a box. I fired it up and decided that i didn't like it, so I decided to make some improvements. I hacked up the original (called amp 1.3) to create amp 1.4 in true sky wired style. Picture 5 shows the original amp all tweaked into something that ROCKED!

OK, it worked......but wasn't going back into the box, so I made a new PC board. It was the ugliest home cooked board I had ever made. I almost trashed it on site, but fixed all the bad spots and testerd it anyway. Pics 6 and 7. I liked what I heard, so I ordered a little Hammond metal chassis from Mouser and stuffed it inside. That's what you saw in the previous posts, and that's why there are no pictures of the guts.....that PCB is ugly. I was going to make a new one, but it works, so.......that was over a year ago and it is currently my only working amp.

So back in the HBAC challenge, this little amp had a pair of bigger twin brothers.....One was a bad a$$, and the other, well don't make him angry.....because you won't like him when he's angry.....he gets really loud.

The twins have been found hiding in a box. Actually one of them lives in a box. The other one grew too tall to fit in the box (it runs EL34's and makes 40 watts). These will be investigated further in the coming months. I might even finish the box if I like the amp. It's been several years since I heard it but it was my favorite of the three.
Attached Images
File Type: jpg Amp1_0_A.jpg (101.0 KB, 20 views)
File Type: jpg Testing_A.jpg (206.6 KB, 19 views)
File Type: jpg Bottom_A.jpg (205.1 KB, 18 views)
File Type: jpg AlmostDone_A.jpg (251.8 KB, 8 views)
File Type: jpg It Rocks_x.jpg (838.2 KB, 9 views)
File Type: jpg NewBoardBottom_x.jpg (802.4 KB, 8 views)
File Type: jpg NewBoardTop_x.jpg (886.2 KB, 4 views)
File Type: jpg UnderTheDeck_A.jpg (242.1 KB, 7 views)
File Type: jpg NoGlow_A.jpg (273.8 KB, 8 views)
File Type: jpg Amp2.X_front_x.jpg (675.4 KB, 9 views)
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Old Yesterday, 02:11 AM   #18
DrAwesome is offline DrAwesome  United States
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My basic layout- is there are glaring issues please let me know- if you want to sing my praises that would be grand
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Old Yesterday, 02:13 AM   #19
DrAwesome is offline DrAwesome  United States
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And this
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Old Yesterday, 05:46 PM   #20
DrAwesome is offline DrAwesome  United States
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Here is a screen shot of the nutone schematic- it lies by the way. There is only one 6.3 volt secondary.

Question- the wiper on “r30” - the 250ohm humbalance- not grounded? I thought it would act as a “virtual center tap” - or is the connection to pin 1 on the 6cu5 having the same function, going to ground through the 220r?
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