Thread-topic: First VTTC - Some help needed choosing tubes
Hello Matt,
I have only built one VTTC, based on a single 833A, so my experience is limited to that coil. There are others on here with more experience with VTTCs so maybe they will respond.
The 811 is a very commonly used tube for VTTCs and can handle decent power. I would stick with a single 811. Dual tubes just complicate matters having to do with matching, dual filament transformers, complexity, etc. I have also heard from other that dual tubes really add little to the streamer length over a single tube. I would use Steve Ward's 833A schematic as a starting point. Just about every VTTC is a variation of it. One thing to incorporate is a way to soft-start the filament. This will extend the life of the tube greatly by avoiding current in-rush to the filament. There are two ways to do it. The best way is with a small variac for the filament. A cheaper way is to have a switchable dual voltage power supply for the filament. A current limiting resistor is used to generate the lower voltage. You then use a 2-position switch to select between the low voltage and nominal voltage supplies.
I wouldn't worry about damaging a tube too much. They are electrically very rugged and can take a lot of abuse. They are not all like IGBTs or MOSFETs which will explode if you look at them the wrong way.
Steve White
Cedar Rapids, Iowa
----- Original Message -----
From: "derstrom8--- via Tesla" <tesla@xxxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Wednesday, December 26, 2018 6:46:43 PM
Subject: Re: [TCML] First VTTC - Some help needed choosing tubes
Apologies for the double-post, but I just came across some 811 tubes as well:
https://www.ebay.com/itm/1x-G-811-811-Audiophile-High-Power-Direct-Heated-Triode-Tube-NOS-Tested/123556917043
I believe these have been used on VTTCs in the past. I'm considering just buying two of them (comes to about $45 delivered) and connecting them in parallel, though they probably won't be matched. I expect I'll have to modify the feedback windings to balance them.
I should also mention I'm shooting for a secondary about 4.5" diameter and probably about 16-18 inches tall.
Thanks,
Matt Lewis
Fairlee, VT
-----Original Message-----
From: derstrom8--- via Tesla <tesla@xxxxxxxxxx>
To: tesla <tesla@xxxxxxxxxx>
Sent: Wed, Dec 26, 2018 5:55 pm
Subject: [TCML] First VTTC - Some help needed choosing tubes
Greetings everyone,
As I mentioned in one of my previous comments in an earlier thread, I am considering building my first VTTC. I have already built a couple of small SGTCs and a DRSSTC, so I think it's high time to try a different route. I have always loved the look of VTTCs and figured now is as good a time as any to give it a go. Unfortunately I have not had much experience with vacuum tubes, with the exception of replacing a damaged rectifier tube in an old oscilloscope to get it running again. Everything I know has been gleaned from different websites around the web. So far I have found Steve Ward's VTTC FAQ page very helpful:
https://www.stevehv.4hv.org/VTTCfaq.htm
As I am in the very beginning stages of this design, not much has been decided with regards to specifications. The only thing I'd like to state off the bat is that I'd prefer to use a small MOT as my plate supply, potentially with a voltage doubler (using a MOC and diode like the microwave oven usually uses, but with the diode flipped around to give me a +HV supply). Beyond that I haven't decided much.
I figured the first place to start would be finding a suitable vacuum tube. Considering I'm completely new at this, I'd like to keep things as cheap as possible. If I blow a tube I don't want to lose a couple hundred bucks. For the past few hours I've been searching eBay for tubes that might work that are within my budget. On Steve's page he says that a variety of tubes will work provided their plate voltage is >1kV or so and are capable of some 10s of watts of plate dissipation. Using these parameters I have found four tubes that I wanted to ask about. Some of them I've never seen used in VTTCs and I'm sure there's probably a reason. I am hoping someone here could give me these reasons as you review the list - I'm looking to learn as much as I can at this point.
Here are the tubes I found:
Eimac 35TG: https://www.ebay.com/itm/Eimac-35TG-Vacuum-Tube-Untested/401670797203At less than $15 delivered this is by far the cheapest I found so far. In Class C operation the specs are as follows:Plate Voltage - 2000V MAX (Key-down conditions. Should I be looking at the Carrier conditions instead?)Plate Dissipation - 50WDatasheet: http://www.tubecollectors.org/eimac/archives/35tg(51).pdf
RCA 6161: https://www.ebay.com/itm/NOS-USN-RCA-CRC-6161-Untested-BUT-Brand-New-Power-Transmitting-Triode/352433703731
This is the next step up in price, at about $30 delivered. It's a goofy-looking tube to me, and I've never seen this type used in VTTC designs. I'm sure there is probably a reason that hopefully someone could explain to me. In Class C operation the specs are as follows:
Plate Voltage - 1600V MAXPlate Dissipation - 250WDatasheet: https://frank.pocnet.net/sheets/079/6/6161.pdf
RCA 845: https://www.ebay.com/itm/1x-VINTAGE-FOTON-GM70-RSA-845-HI-END-POWER-TRIODE-USED-TESTED-100-60-s/302964678385This is the third cheapest option, at about $40 delivered. Datasheet specs Class A and Class AB operation, but no Class C. Does this make it unsuitable for a VTTC?Plate Voltage - 1250V MAXPlate Dissipation - 100WDatasheet: https://frank.pocnet.net/sheets/049/8/845.pdf
Amperex 5868: https://www.ebay.com/itm/Used-Amperex-5868-RF-Power-Triode-Tube-for-Modulator-Oscillator-or-Amplifier/372542462646This is by far the most expensive, at about $100 delivered. This is pretty much blowing my budget, but is also probably the most promising. Considering the specs I expect this one would be able to take quite a bit of abuse, assuming the rest of the ratings are acceptable for a VTTC. In Class C operation the specs are as follows:Plate Voltage - 4000V MAXPlate Dissipation - 450WDatasheet: https://frank.pocnet.net/sheets/089/t/TB4-1250.pdf
So these are the tubes I'm looking at currently. If anyone has thoughts or recommendations for any of the above tubes, or good sources for other cheap tubes, please let me know. Once again I'm in the learning stage, so the more information you can provide, the better!
By the way, a great resource for tube datasheets is https://frank.pocnet.net/sheets0.html , if anyone is interested. It has been immensely helpful to me so far.
Thanks,
Matt LewisFairlee, VT
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxxxxxx
https://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxxxxxx
https://www.pupman.com/mailman/listinfo/tesla
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxxxxxx
https://www.pupman.com/mailman/listinfo/tesla