Aikido Transformers - Good Choice?

[slhijb]

Starting my very first ever electronics build and have selected the Aikido Octal All-in-One LSA/HPA Line-Stage preamp with 6SN7 input & output tubes (Janus Shunt regulator).

Octal LSA/HPA PCB

Have read about the various Aikido configurations, safety and electronics in general and am ready to order the heater and B+ power transformers. Want to make sure I‘ve made good choices. Look good?

1. Low Voltage Regulator - HEATER Power Supply: EDCOR PWRC12.6V3A-1
Power transformer from 120V, 60Hz. to 12.6V at 3A center tapped.
EDCOR - PWRC12.6V3A-1

2. High Voltage Regulator - The B+ Power Supply: EDCOR WPWR050
Tube Amplifier Power Transformer - 120V, 60Hz. to 250V at 200mA and 6.3V at 3.5A.
EDCOR - XPWR050

 

[Arnulf]

Why first transformer ? You can run the heaters from the second transformer.

 

[slhijb]

Thanks Arnulf for the reply. Trying to get a grip on basic electronics, so please be patient. Want to use 2 separate transformers. The way I understand it is with 2 transformers the heater transformer can be switched on first, in the standby mode so to speak, to allow the tubes to warm-up prior to turning on the main power supply. This helps reduce the shock to the tubes from the initial power surge experienced when switching on a single transformer. Having 2 transformers also reportedly isolates power supply spikes sometimes experienced in the main power transformer from the heater transformer.

Here are the stated power supply requirements identified in the Aikido Octal All-in-One description (the first link above).
“The power supplies require an external power transformer with three secondary windings, 5Vac for the tube rectifier, and a heater winding such as 6.3Vac or 12.6Vac and between 200Vac to 275Vac (400Vac CT to 550Vac CT) for the high-voltage power supply.”

I want to make this a first rate preamp. Can you recommend a better B+ & heater transformer combination for this application? Thanks again.

 

[ChrisA]

Thanks Arnulf for the reply. Trying to get a grip on basic electronics, so please be patient. Want to use 2 separate transformers. The way I understand it is with 2 transformers the heater transformer can be switched on first, in the standby mode so to speak, ...

The way "everyone" does this for the last 60 years is to make a "standby" switch. This is just a simply an on/off (SPST) toggle switch on the B+ voltage. I'd place it between the rectifier and the first input filter cap.

This way you still have two switches but only one transformer

Here is a good on-line article that talks about standby switches
The Valve Wizard

 

[Arnulf]

to allow the tubes to warm-up prior to turning on the main power supply. This helps reduce the shock to the tubes from the initial power surge experienced when switching on a single transformer. Having 2 transformers also reportedly isolates power supply spikes sometimes experienced in the main power transformer from the heater transformer.

All true, at least in theory, provided that you use DC heater supply (if you don't, the point about noise from heater supply is completely irrelevant). Thing is, Broskie's Aikido is supposed to have quite good power supply noise rejection so whatever noise could end up inductively coupled in B+ supply should therefore be eliminated as well. This is why I wouldn't bother with a separate heater transformer, unless I was using cheap isolation transformers for the B+ (over here we have 230V mains so we end up with 230V from isolation transformer, quite useful for tubes and isolation transformers are sometimes found at scrap prices and usually come in high ratings, tens or hunderds of VA).

Now as for heater warmup, ChrisA has covered that for you if you really want to pursue this route. This is, in my opinion, unnecessary; I don't believe cathode stripping to be an issue with tubes such as yours, plus high voltage across the B+ switch might lead to problems (arcing). If your supply uses a choke the arcing problem can only get bigger. In addition to that you're trading another potential problem (cathode poisoning) for cathode stripping.

If I wanted to delay B+ for 10 seconds (until the cathode warms up) I'd use a suitable time constant in regulator reference - this is what I normally do.

I want to make this a first rate preamp. Can you recommend a better B+ & heater transformer combination for this application? Thanks again.

If it was me I'd use the second transformer alone. It's more than up to the task of supplying all the power you need.

I think two transformers might be a solution in search of a problem, rather than solution of an existing problem.

 

[slhijb]

Arnulf and Chris, thanks so much. A single transformer it is. Sounds simpler and cheaper... Good deal.

Also, am planning on using an SPST ON/OFF switch connected per Chris’s recommendation. Will place an LED indicator “ON” light with a series resistor across the mains supply per the first photo in Chris's recommended article, the Valve Wizard.

 

[Arnulf]

Why not use a neon lamp, it uses less current, it is made of glass rather than plastic and is considerably more retro than LED 😀 You can buy these with suitable resistor already attached.