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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 13th August 2019, 04:49 PM   #451
zman01 is offline zman01  Bangladesh
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After a 14 year run, the TSE must DIE!
Quote:
Originally Posted by itsikhefez View Post
Per the BOM notes:
"2) R2 MUST be 100 ohms for 45s and 2A3 tubes to provide 2.5 volts. R2 MUST be 300 ohms for 300Bs to provide 5 volts. Measure the filament voltage BEFORE installing any tubes!"
This part needs to be how many watts?
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Old 13th August 2019, 05:49 PM   #452
Mr_Zenith is offline Mr_Zenith  United States
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After a 14 year run, the TSE must DIE!
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I assumed "or higher" meant .22 or .47 but seems like you meant higher voltage rating.
He did. As George explained, mucking about with the capacitance value for the coupling caps can wreak havoc with the frequency response and/or cause system instability (bad). Their voltage rating is 450 V minimum to prevent breakdown of the dielectric (really bad). Regardless, the specified 0.1 uf cap was chosen carefully and will yield plenty of bass.

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This part needs to be how many watts?
I believe I used a 1/2 watt resistor because it's what I had on hand, but you could likely get away with 1/4 watt. It creates a voltage divider to set the output for the regulator, so it doesn't see any real current.

Last edited by Mr_Zenith; 13th August 2019 at 05:57 PM.
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Old 13th August 2019, 06:06 PM   #453
Tubelab_com is offline Tubelab_com  United States
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Quote:
you meant higher voltage rating.
Yes. The cap can see B+ voltage on the driver end before the tubes are warmed up, and bias voltage on the other end which can be 80 volts or so on a 2A3 or 300B that is run at a high B+ voltage. A cap rated for 450 volts is the minimum I would use here unless you know that your amp does not go higher. A 45 amp running about 300 volts could use a 400 volt cap, but an amp running one of those pricey mega 300B's on 450+ volts probably should use a 600 volt cap.

If a .22 cap works fine in your application, use it. One of my old TSE's had 1uF caps in it because I had a box full of 1uF "Wonder Caps" (remember that fad from the late 80's). They worked fine, and are still in the amp.

Quote:
This part needs to be how many watts?
a 1/4 watt resistor is fine. It sees less than 50 milliwatts.
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Old 13th August 2019, 06:14 PM   #454
itsikhefez is offline itsikhefez  United States
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After a 14 year run, the TSE must DIE!
Quote:
Originally Posted by Tubelab_com View Post
If a .22 cap works fine in your application, use it.
Which measurement can I perform with an oscilloscope or sound card that would indicate whether the larger cap is an issue or not ?
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Old 13th August 2019, 11:24 PM   #455
Mr_Zenith is offline Mr_Zenith  United States
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After a 14 year run, the TSE must DIE!
I'm not an authority on oscilloscope measurements by any means, so I'll leave that to others who know better. But I will say that sometimes the effects of coupling caps can be obvious. I once tested a breadboarded amp about 10 years back that sounded "funny". When I removed the speaker grilles I could see one of the woofer cones was pumping in and out at about 1/2 Hz. The oscillation disappeared when the 1 uF coupling caps (the schematic I was following called for them) were replaced with 0.22 caps.

George's remarks are absolutely correct though, and my apologies if I sounded like I was speaking in absolutes or talking out of turn. I was replying via my phone from the cab of a parked truck on a 90 F day.

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(remember that fad from the late 80's)
I've never heard of them as I was in my 20's then and far too busy being young and stupid. Apparently they're still available, though the selection is severely limited (I suspect they're likely the last remaining NOS). Now that I'm old and stupid, I find that orange drops suit me just fine. Usually.
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Old 14th August 2019, 01:58 AM   #456
zman01 is offline zman01  Bangladesh
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After a 14 year run, the TSE must DIE!
Based on bill of materials shared by Tubelab in post 440 of this thread, here you go with the link for the Mouser shopping cart:

Mouser Electronics

I have also added the Triad C14-X choke in this shopping cart - please change according to your preference/requirement.

The tube sockets listed in the bill of materials are not included in this shopping cart - you will need to order separately.

Hope this helps.

Last edited by zman01; 14th August 2019 at 02:12 AM.
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Old 14th August 2019, 01:25 PM   #457
Tubelab_com is offline Tubelab_com  United States
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Quote:
I had a box full of 1uF "Wonder Caps" (remember that fad from the late 80's)......I've never heard of them as I was in my 20's then and far too busy being young and stupid. Apparently they're still available,
In the 60's I built tube stuff because parts were free, just rip what you need out of some dead consumer electronics, or buy some cheap military surplus gear and gut it. In 1973 I started a 41 year career at Motorola where silicon was free just by filling out a sample request form, so I built solid state stuff into the late 80's and early 90's.

I started building tube stuff in the early 80's but it was mostly guitar amps. HiFi tube amps came in the late 80's when the audio BBS's (pre WWW internet bulletin boards) were ruled by "guru's" and "shaman's." The engineer in me cried BS, but when I saw a big bag of these caps "removed from power supplies" on Ebay for cheap, I bought them. Most were Rel-Caps, and a few were Wonder Caps. I believe that they are the same, just the marketing and price were different. They were supposed to contain three identical caps inside in parallel for reliability, but I never cut one open.......still have most of them!

I stuck them in both of the original DIY TSE breadboards. One of those breadboards still resides in my 845SE amp where it functions as the driver. The other was in my Lexan TSE. I swapped it for a production quality board when I rebuilt the amp after it got flooded with dirty water during a hurricane in 2006.

That board and my collection of Expenso-Caps are pictured here.

Quote:
Which measurement can I perform with an oscilloscope or sound card that would indicate whether the larger cap is an issue or not
I doubt that you will have any issue since you are running a rather low power level in an amplifier that does not use any feedback.

The issue arises when your source can put a lot of very low frequency information into the amp AND the power level is high enough that this can cause saturation in the OPT. The coupling caps are often sized such that frequencies below what the OPT can handle are rolled off. You could test the THD at lower and lower frequencies and at power levels your amp will actually see to verify that this isn't happening, but if you don't hear distortion on bass notes when listening, the actual numbers don't matter.

Quote:
amp about 10 years back that sounded "funny". When I removed the speaker grilles I could see one of the woofer cones was pumping in and out at about 1/2 Hz. The oscillation disappeared when the 1 uF coupling caps (the schematic I was following called for them) were replaced with 0.22 caps.
The other issue occurs when you stuff big coupling caps into an amp that runs GNFB, or any feedback wrapped around a transformer. Here the cap should roll off the low frequencies below the point where the OPT starts shifting the phase of the low frequencies. If not the negative feedback can be phase shifted into positive feedback making the amp oscillate at a low frequency.

This can be destructive in a high powered amp......I learned this the hard way with these same caps, some 1990 vintage Chinese KT88's and a guitar amp quality OPT specced only to 80 Hz. It IS possible for a poorly constructed tube to shatter from a violent lightning storm inside.
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Old 14th August 2019, 11:05 PM   #458
Mr_Zenith is offline Mr_Zenith  United States
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After a 14 year run, the TSE must DIE!
Quote:
Based on bill of materials shared by Tubelab in post 440 of this thread, here you go with the link for the Mouser shopping cart:
Hi zman01, thanks for taking the trouble to create that! I wish it'd been there when I placed my order, but it'll no doubt help others.

Considering George's comments in Post 420 - and on my own experience - I'd suggest changing R5 to at least a 2W part lest others experience the "thrill" of smoke and failing bias.

I got lucky, but other builders might not...

Last edited by Mr_Zenith; 14th August 2019 at 11:08 PM. Reason: Space is the place!
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Old 15th August 2019, 02:03 AM   #459
Tubelab_com is offline Tubelab_com  United States
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I still haven't solved the R5 puzzle. I get some 1 watt parts that don't blow when charging a board to 450 volts, while some 2 watt parts fry. Repeat the same experiment a day later and get different results. More resistors on the way. This time I'm looking at different resistance values. 270 ohm was the optimal part for the board when I was running 45 at 320 volts. That old Digikey part never caused trouble though and the old stock I have is good. I got some new stock coming to verify.
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Old 16th August 2019, 09:38 AM   #460
baggerbole is offline baggerbole  Germany
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To all who have problems with 5842 procurement,
there is an adress in Germany with limited stock:
Frag Jan zuerst --- Ask Jan First
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