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Old 17th January 2019, 08:14 AM   #1
jgx is offline jgx  Spain
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Default Some doubts about material building a Tubelab SE based on 2A3

Hello everybody. This is my first time in this forum, I have read a lot, but never I had posted nothing.

Recently I have purchased a Tubelab SE PCB. I'm working in buying materials, transformers. I had thought the next options:

-Output transformers: Lundhal 1664

-Power transformer: In this topic I have a doubt between Hammond 370HX (George recommend Hammond 270HX, is the same but with 125V in the primary, I live in Europe and I need 240V in the primary, so the equivalent is 370HX) or Hammond 302AX that Hammond recommend for 2A3 tubes. The main difference (and this is my doubt) is that 370HX works at 550V CT and 302AX work at 600V CT. Other important difference is that 302AX provide two secondaries at 2.5V@2.5A and 370Hx just one secondary at 5V CT@3A. I think that maybe it is not enough current to feed the filaments of the 2A3 tubes, but George recommend 270HX.

Any comments, suggestions about the choice of power transformer between these two options?


Thanks in advance!
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Old 17th January 2019, 02:52 PM   #2
Tubelab_com is offline Tubelab_com  United States
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There are some issues with running 2A3's in the TSE. Each 2A3 draws 2.5 amps of heater current for a total current draw of 5 amps. The filament regulator chip is specified to operate 5 amps maximum. The heater current drawn by a tube can vary quite a bit from tube to tube, and I have seen some tubes that draw nearly 3 amps each. If a pair of tubes that need nearly 3 amps each are used in the amp, the filament regulator will shut down, or regulate poorly resulting in hum. There is a 7 amp version of this regulator, but I don't have any, so I don't know how well it works.

There have been some good sounding TSE's with 2A3's and there have been some issues with hum. The hum problems seem to be caused by low line voltage, and the 300 series Hammonds have several primary taps, so that a suitable choice could be found.

Another trick needed with 2A3's is to add an off board cap in parallel with C1 to reduce the ripple going into the regulator at the high currents seen with 2A3's. An additional 20,000 to 30,000 uF is about right.

The power transformer must be able to supply at least 6 amps of 6.3 volts with a center tap.

Quote:
370Hx just one secondary at 5V CT@3A. I think that maybe it is not enough current to feed the filaments of the 2A3 tubes, but George recommend 270HX.
The 370HX has a 6.3VCT at 6 amp secondary in addition to the 5 volt winding.

A 600 volt secondary will usually result in a B+ of about 350 to 370 volts, which is above spec for a 2A3.
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Old 17th January 2019, 06:53 PM   #3
jgx is offline jgx  Spain
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Thanks George for your fast response. OK, I understand that with the Hammond 370HX I have enough current with the 6.3V/6A secondary. So the best choice would be the Hammond 370HX.

I don't understand why a low line voltage can cause hum. In Spain we have 230V AC more or less, so I would have to use de 220V primary configuration in the power transformer to get a greater line voltage?. Moreover, I'm concern with the possibility of get hum noise using 2A3 with the 5A regulator. I have a 300B based amplifier and I want build other amplifier with different output tubes. If I use a big dissipator (as you have done), maybe I will remove this hum? I had thought to buy two JJ 2A3 tubes (2.5A/250V), but now I'm confused.

Someone could provide a configuration (tubes/power transformer, anything...) to use TSE PCB with 2A3 tubes? Or maybe I should think to change the output tubes choice, maybe 300B? Today is dificult find trust 45 NOS tubes.. Any advice?

As you can see I'm so confused! But it is funny!!
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Old 17th January 2019, 07:47 PM   #4
Tubelab_com is offline Tubelab_com  United States
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Quote:
I don't understand why a low line voltage can cause hum.
The TSE uses DC heating so that there is no hum from AC filaments in the output tubes. The transformer's heater voltage is rectified, then filtered and then regulated to provide pure DC for the output tube heaters.

The 2A3 needs 2.5 volts for its heater. There is some residual AC ripple on the rectified and filtered DC voltage which is removed by the voltage regulator. The regulator needs some headroom to do it's job, which is not always available if the line voltage is low.

The 300 series Hammopnds have taps for 220, 230 and 240 volts, so you can chose which one works the best for you. It will probably be the 220 or 230 volt tap.

When I designed the board the biggest cap that was available for C1 was 15,000 uF at 10 volts. It is possible to add a second cap in parallel with the cap in the C1 spot on the board to boost the headroom by up to half a volt.

2A3's should work fine in your board if you use a multi tapped power transformer, and add an extra cap in parallel with C1.

I am currently testing alternative chips to use for the regulator chip in the TSE. Some test results, and pictures of the line voltage testing I use will be posted in the TSE must die thread as soon as I get it ready. It may help your understanding.
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Old 17th January 2019, 09:11 PM   #5
jgx is offline jgx  Spain
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Thanks George. Probably I will buy the Hammond 370HX. I will be awaiting your ideas in the post TSE must die. Any suggestions about the type and capacity of the capacitor to be in parallel with C1.

Last question, if I will use a 600V CT power transformer to feed 2A3 the B+ will be 350 to 370 V as you said before, but could I increase the value of R4 to reduce the voltage until a safe value of B+ for the 2A3 tube? I would prefer buy the Hammond 302AX because it has more secondaries and is more useful for future projects, the only problem is they it has 600V CT. What do you think about that?

Thank you very much!
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Old 17th January 2019, 10:56 PM   #6
w5jag is offline w5jag  United States
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The 302AX is a good choice for a simple, traditional, 2A3 type amp - it has a 2.5 volt secondary for AC filament voltage for each power tube, and a center tap on each of those secondaries for connection of the necessary cathode resistor to bias the amp, and bypass cap.

Also, the higher voltage secondary is designed to take into account the voltage drop across the cathode resistor biasing the amp - which will be 50 to 60 volts.

The TSE is a more sophisticated, non traditional, type of amplifier - it uses DC on the filaments (cathodes) of the power tubes, and uses a separate negative voltage for the bias and the MOSFET power drive.

Win W5JAG
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Old 18th January 2019, 01:47 AM   #7
Tubelab_com is offline Tubelab_com  United States
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I now have a TSE running on the bench with 2A3's a Hammond 372HX 600 volt transformer. The B+ is anywhere between 340 and 350 volts depending on line voltage. I get 345 volts B+ with 120 volts AC line voltage applied to the 120 volt taps.

I am using cheap Chinese twin plate 2A3's which are clones of the old RCA twin plate tubes. They don't seem to mind the high voltage, but I found an easy way to drop the B+, simply use a 5U4 rectifier instead of the 5AR4. I pulled a GE 5U4GB out of a working guitar amp and stuck it into the TSE. It reduced the B+ from 345 volts to 310 volts. Easy to do, no electrical changes needed.
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Old 18th January 2019, 07:39 AM   #8
jgx is offline jgx  Spain
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Thank you very much for your advice! Yesterday I watched your post about use 600V PT in 2A3. So finally, I will bought the Hammond 302AX that, as said by w5jag, the power transformer used for a "usual" 2A3 based amp. Now I will use it with TSE, changing the rectifier tube, using 5U4GB and adding a capacitor in parallel with C1. Moreover yesterday I ordered 3 regulators PQ5EV7 from UTSOURCE (and 4 2SK3563), expect that don't be a fake components.

What is the manufacturer of Chinese twin plate 2A3's which are clones of the old RCA twin plate tubes?

Other question, I have a doubt regarding B+ jumper, I'm going to use the PCB as a SE amplifier, with the transformer that I have mentioned. Must I connect the jumper between BPLUS and DRV_B+? I don't have clear the explanation in the Tubelab webpage, watching the PCB and the schematics the only way to connect MOSFET drain with current source is connect this jumper, isn't it?


Sorry for the huge amount of question

I Keep on running
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Old 18th January 2019, 02:49 PM   #9
Tubelab_com is offline Tubelab_com  United States
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Quote:
What is the manufacturer of Chinese twin plate 2A3's
Mine are 15 year old Shuguang tubes. I have no idea if they still make them.

Quote:
Must I connect the jumper between BPLUS and DRV_B+?
Yes. All boards should get that jumper except those used with external power supplies.
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Old 18th January 2019, 02:54 PM   #10
jgx is offline jgx  Spain
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Thank you very much George!
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