45/2a3 amp primary load imped. with 46 tube

[kevtn8]

Hi, fellow Bottlehead users and moderators. My name is Kevin, I'm posting this here because I'm a big fan of tubes and I have a nice 45/2a3 SET amp custom made from a highly respected German designer. I was wondering if anyone very knowledgeable with tube circuits and transformers can clear this up for me.

Basically my amp has speaker taps for 4, 8, 16 ohm. However the designer told me that this is only for 2a3 tube. When I'm using 45 tubes ( switch activated), he told me the speaker taps become 8, 16, 32 ohm due to the 45 having double the plate resistance of the 2a3. My amp power supply has also been modified by the designer to also be able to use 46 tube ( w/ tube adaptor).

My output transformer are from Tango ( XE-20S) which has primaries of 2.5k, 3.5k, 5k activated depending on which secondaries are hooked up.

Following up above on the relative impedance difference of the 46 tubes compared to the 2a3, the speaker taps would therefore be roughly 12, 24, 48 ohms when 46 tube is used.

Since my speakers are 16 ohms, can anyone confirm for me if the 46 tube is properly loaded on my amp when I'm using the 12 ohm reflected speaker tap ? Thank you very much in advance.

Kevin

[Paul Joppa]

The best load impedance for a tube depends on the plate voltage and current, as well as the plate impedance. The "usual" output transformer impedances for a particular tube are just the examples shown in the data sheet - they do not cover all the possibilities. For examples, you can look up the Western Electric data sheets for the 300B. They usually include a large list of operating points (voltage, current, and load impedance) - voltages range from 200 to 450, current from 30mA to 80mA, and load impedance from 1500 to 6000 ohms.

A competent designer will have chosen operating points that work well, at least according to the designer's idea of the relative importance of things like power, distortion, damping factor, longevity of the tubes, etc.  I imagine that your designer just mentioned one of the variables because anything more would take too long to explain.

Here's a link to a data sheet for the Tango XE 20S - note that there are many more than three secondary impedance taps:

http://tango-trans.com/wordp/wp-content/uploads/catalog/XE-20S.pdf

So the designer had available many options for matching the needs of the tubes - you just have to trust that appropriate combinations of operating conditions and transformer connections were chosen.

[kevtn8]

Hi Paul,

I can tell you based on his standard 45/2a3 amp with power being 2 watts for 45 and 3.5 watts for 2a3 , that the main mod done to the power supply was lower plate voltage to 250 volts to accomodate the 46 tube. I had instructed him to keep the 46 within the max rated spec and that my main concern with this amp was best sound quality, not maximum power. 

My particular version of his 45/2a3 amp puts out about 1.7 watts for 45 and 3 watts for 2a3. He also told me that the 46 is within spec but on max allowed voltage so this confirms 250 plate voltage.

Its interesting that this Tango transformer had that many secondary options. I only requested 4, 8, 16 ohm speaker taps.

I only use the 46 tube on the 45 selector switch obviously and it sounds great so I'm wondering if this is a good sign that the 46 tube is somewhat loaded in the correct ballpark. I've only tested the 46 on the reflected 12 ohm tap ( which is actually 4 ohm speaker tap) as this makes the most sense on my 16 ohm speaker.

One knowledgeable circuit guy I asked on Youtube calculated the turns ratio of this Tango transformer at 17.7:1. Since I'm using 3 possible tubes on this amp I'm a bit confused. Is the turns ratio a constant value regardless of output tube or would the differences in plate impedances among these tubes change the turns ratio and thus the loading of these tubes on my 16 ohm speaker ? Thanks

Kevin

[Paul Birkeland]

The turns ratio changes depending on which secondary taps you use. 

[kevtn8]

Thanks Paul. I've calculated some possible reflective loads on the 46 tube in my amp with the 16 ohm speaker.

5k/ 12 ohm reflected ( 4 ohm tap) x 16 ohm  = 6,667 ohm

2.5k / 12 ohm reflected( 4 ohm tap) x 16 ohm = 3,333 ohm

5k / 4 ohm tap x 16 ohm = 20,000 ohm

2.5k / 4 ohm tap x 16 ohm = 10,000 ohm

Since subjectively it sounds great from bass through treble and we know from data sheet that the 46 wants around 6400 ohm load at 250 plate voltage can we assume that on my system , its most likely the very first load above ? My second choice would be the very last load at 10k. What do you think or am I missing something here ?

Thanks
Kevin

[Paul Birkeland]

Was the manufacturer of your amplifier not able to answer these questions?

[kevtn8]

Hi Paul,

The designer is German and unless its a technical warranty issue, he felt that it was not worth getting into too much details. The most he would tell me was what I posted in first post. He stressed that since no speaker ever has constant load across its total frequency range that it was moot to even talk about it and that if 46 sounds good on my speakers then thats what I should go with.

The only reason I'm curious for my case is because I wanted to know whether I should abandon using the 46 in this amp ( to focus on 45 and 2a3 ) or whether I should invest in an impedance modifying device such as the Paul Speltz Zero Autoformer to further raise up impedance of my speaker.

Kevin

[Doc B.]

he felt that it was not worth getting into too much details

It's unfortunate that the manufacturer of the amp you bought does not want to answer your questions. Our focus here is support for our customers regarding Bottlehead products. I think you would probably do best to take your questions to DIY Audio, Audio Asylum or similar forum where the focus is on hobbyists working up their own designs and modifying manufacturer's existing products. You may find people there with experience with the 46 tube.