Tubelab SE (300B + Softone RW-20 OPT) Wiring

[Junior]

Hi,
I had built myself a Simple SE and had been enjoying since.
Now is time for another built and is going to be Tubelab SE.

I had acquired a pair of Softone RW-20 OPT, which allow Non-Inverted and Inverted connections. (Softone RW-20 Single-Ended Audio Output Transformer).

Question 1:
What is the meaning of Non-Inverting and Inverting?
Does it mean the output signal to the speaker is inverted/not inverted just as in OP Amp?

I had encountered problem with the connection/wiring for the RW-20 OPT.
(Between T2-1, T2-2, T3-1, T3-2 and the RW-20 OPT to speaker output).
I had come out with the connection/wiring (on paper) to the very best of limited technical understanding. I hereby need to consult the experts here to check/confirm the connections.

Question 2: (see attached picture)
Please do help in checking my connection/wiring as shown in the attached pictures.
Do correct me for any wrong connections.
PS: T2-1 & T2-2 – Non-Inverted Connection @2700 ohm load.
T3-1 & T3-2 – Inverted Connection @5000 ohm load.

Question 3: (see attached picture)
I had added a switch so that I can switch between the 2 loads type (2.7k or 5k).
Is that the correct way to do it?
If yes, what are the suitable types of switch to be used in term of specification? (500V? 15A?)



Question 3:
The Softone RW-20 comes with ultra-linear tab for 5000 ohm load.
Am I able to make use of the UL tab for Tubelab SE using 300B?
If yes, how do I make the connection?


Thanks in advance.

[tomchr]

It looks like your transformer has two UL taps, one for each load impedance. If you choose to use UL and switch the load impedance with a switch, I suggest that you switch to the appropriate UL tap also. This could be done with an DPDT (double-pole, double-throw) switch. The switch should be able to handle at least 100 mA and handle the B+ voltage. Honestly, though, you won't be switching the load impedance or UL tap while the amp is running so you could as well connect the appropriate taps on a terminal strip and call it good. I like using the "Euro-Style" terminal blocks for this.

Inverting vs non-inverting: Notice the little dots on the transformer diagram? That means the voltage on the white speaker wire is in phase with the voltage at the orange (B+) wire. I.e. if the voltage at the orange wire is positive with respect to any of the other primary connections, the white wire will be positive with respect to the black. So your non-inverted connection is correct. For the inverted connection, just swap the two speaker wires (white-black). Don't mess with the primary connections.

UL is for pentodes only. The 300B is a triode. There's nowhere to connect the UL tap so just leave it floating (tape off the end or connect it to an unused terminal on a terminal strip).

~Tom

[Ty_Bower]

Tom answered your questions well... I have nothing to add. But, I have heard good things about the Softone RW-20 output transformers. I'm interested to hear your listening impressions after you've got the amplifier completed.

[Evenharmonics]

Originally Posted by Junior

(Softone RW-20 Single-Ended Audio Output Transformer).

Are you going to be augmenting bass with subwoofer? The frequency response shows considerable roll off below 100Hz.

[kevinkr]

Originally Posted by Evenharmonics

Are you going to be augmenting bass with subwoofer? The frequency response shows considerable roll off below 100Hz.

Presumably he will be driving it with a DHT since the TSE uses those, based on the charts provided the response will only be down 1dB @ 20Hz for an rp of 600 ohms. Best performance for the 2A3 will probably be on the 2.7K tap, and for the 300B either could be used, but I'd go for the 5K tap.

Note the plate current ratings for each impedance tap - on the 5K tap 60mA is the maximum..

[Junior]

hi tomchr,
for my build with Softone RW-20 as OPT i will not be able to use the UL tab as you had mentioned that the 300B is a triode whereas UL is for pentodes. and thanks for the explanation of Non-Invert and Inverting.

however, glad that i can still make use of a switch to switch between 2700 or 5000 ohm load impedance.

hi Ty_Bower,
i purchase the RW-20 with no research.
only after the deal then i do a search in the net and not too bad a review though not many users. hope it turn out to be good.
it going to take quite a while for my TSE build before i can post my listening impression.

hi Evenharmonics,
will not complement the TSE with sub-woofer.
in fact, the Simple SE that i built is already very impressive for a start and i assume the TSE will definitely be a level higher.

hi kevinkr,
i will be using a switch so that i can switch between the 2 load impedance.
in fact, i am still keen in building the TSE to be universal, which explain why the switch.

hi all,
since i had settled with the wiring up of the OPT now is the power transformer for 300B and/or 2A3.

can anyone suggest to me the suitable power transformer from Hammond (universal primary voltage - since my country is 240V 60Hz) for my TSE. Reason why Hammond is easily available locally. used 374BX for my tubelab simple SE built.

is 370HX suitable for my application for 300B?

and with no major changes in the wiring (except for the load impedance switch) can 370HX also be used for 2A3 (choose 2.7k ohm load)?

am a step closer to finalize my build.

[Evenharmonics]

Originally Posted by Junior

hi Evenharmonics,
will not complement the TSE with sub-woofer.
in fact, the Simple SE that i built is already very impressive for a start and i assume the TSE will definitely be a level higher.

You may not need a subwoofer depending on the speakers you are using.

can anyone suggest to me the suitable power transformer from Hammond (universal primary voltage - since my country is 240V 60Hz) for my TSE. Reason why Hammond is easily available locally. used 374BX for my tubelab simple SE built.

I used to have Hammond 272JX for PT of my TSE but it buzzes like most Hammond PTs do. I switched to Toroidal PT shown here and have been happy with the results so far.

[kevinkr]

Originally Posted by Junior

hi tomchr,
for my build with Softone RW-20 as OPT i will not be able to use the UL tab as you had mentioned that the 300B is a triode whereas UL is for pentodes. and thanks for the explanation of Non-Invert and Inverting.

however, glad that i can still make use of a switch to switch between 2700 or 5000 ohm load impedance.

<snip>
and with no major changes in the wiring (except for the load impedance switch) can 370HX also be used for 2A3 (choose 2.7k ohm load)?

am a step closer to finalize my build.

For best results the 300B is going to want to be run with a plate voltage of 350V - 400V with fixed bias, but the 2A3 should not be run much over 300V and 250V is probably best unless using a super 2A3. This means you will need to make provisions to reduce the plate voltage when you swap tubes. Something to think about. No suggestions yet on the best way to accomplish this.

In terms of power transformers I would look into something other than Hammond, although the 3 series transformers should be OK. How about Silk from Thailand?

[Evenharmonics]

Originally Posted by Junior

300B and/or 2A3.

Some 2A3 tubes have current rating (draws more than its spec) problem with TSE design. Sovtek 2A3 is one example which I started with but ended up switching to 300B. Seems like it's either 45 or 300B around here for TSE amp.

[tubelab.com]

Some 2A3 tubes have current rating (draws more than its spec) problem with TSE design.

This is true. The filament regulator in the TSE is rated for 5 amps. A 2A3 SHOULD draw 2.5 amps. The filament regulator has ample headroom to put out about 5.5 to 6 amps depending on temperature, line voltage, the power transformer, and the particular chip in question. I purchased the exact Sovtek 2A3's discussed above and found that they work in some TSE's and not in others. I have not seen an issue with old RCA tubes. You really need a power transformer with an 8 amp rating on the 6.3 volt winding to run 2A3's without an issue. The transformers with a 6 amp rating are marginal causing the filament regulator to drop out of regulation leading to hum.

For best results the 300B is going to want to be run with a plate voltage of 350V - 400V with fixed bias, but the 2A3 should not be run much over 300V and 250V is probably best unless using a super 2A3. This means you will need to make provisions to reduce the plate voltage when you swap tubes. Something to think about. No suggestions yet on the best way to accomplish this........is 370HX suitable for my application for 300B?

The 300B likes 350+ volts. The maximum spec for the 2A3 is 300 volts. The 300B needs a 4 amp filament rating while the 2A3 needs 8. It is nearly impossible to find one power transformer to use with both tubes. The new 370LX should work but it isn't available here in the US yet. It will generate about 320 volts of B+ which will work with most 45's 2A3's and 300B's. With 320 volts a 300B will only make about 6 watts. My Lexan amp uses an Allied 6K56VG which is a Hammond made 550 volt transformer similar to the 370EX. it works with the 45 and 300B but doesn't have the filament capability for 2A3's. I get 2WPC with 45's and 5 WPC with 300B's using 5K OPT's for both.

I have successfully used the 272JX (a 120 volt 372JX) with 2A3's and 300B's, but this is the transformer that Evenharmonics has trouble with. It also runs the 2A3 well over it's maximum rating with the B+ at about 355 volts. None seem to mind the extra voltage even the old RCA's (all dual plate versions). That said I'm sure that there is a 2A3 out there that will blow up or run away at this voltage, so it is not recommended. The 300B works very well at this voltage, but some people prefer more B+. The 376X will get the B+ up to near 400 volts but is not suitable for the 2A3 and good ventillation around the PC board is needed for this voltage level.