Need help with output transformer for tube mic pre

mbf90 · Jan 8, 2018

Hi,

I have a Hamptone HVTP2 tube mic pre amp. I want to try some vintage altec transformers in place of the Altran 4:1 output transformers. Specifically, the Altec 15095(a). The Altran’s are nice but too clean for the sound I want from this Pre amp. This mic pre was featured in a tape op article and the designer, Scott Hampton, actually mentioned using the Altec 15095 for the output yielded great results.

My question is in regards to the difference between the Altec 15095 and the 15095a. Scott used the (a) version in his prototype that he mentioned in the article. I want to know if I can safely use the 15095 non (a) version in this design. The (a) version Scott used has an independent grounding pin, while the non (a) version’s grounding pin is tied to the primary. From what I can tell by the looking at the schematic for the Hamptone, it’s output transformers have a grounded primary. Here is a link to the schematic. https://tapeop.com/_m/photologue/photos/vacuum-f1.jpg

Am I correct that I can use the grounded primary 15095 without a problem?

Thanks,

Matt

DaveP · Jan 9, 2018

From your schematic, the primary has a grounded primary so problem solved.

When manufacturers make slight changes they tend to call them a or b versions, they are hardly ever totally different products as that costs too much money in tooling costs. In the Altec case it looks like they have just added another wire instead of automatically grounding the primary.

DaveP

mbf90 · Jan 9, 2018

Thanks Dave.

That’s what I figured. It looks like the 15095a just allows more flexibility with grounding. I was just a little worried because I read on another forum that it could be dangerous for the equipment to use the original grounded primary version with certain designs. However, as you pointed out, the hamptone schematic shows a grounded primary in the output.

Matt

DaveP · Jan 9, 2018

Your schematic is not much different from the audio path in an LA-2A compressor (but without the feedback path) so it should sound pretty good. Check it out:

They both share a White Cathode Follower (WCF) for the output stage and twin triode for the input.

DaveP

mbf90 · Jan 9, 2018

Yea, it sounds fantastic. Best sounding mic pre I have had the pleasure of using. The output stage is interesting as it does look just like a white cathode follower, which I have seen called a push pull output. However, the designer told me it is not push pull and does not split the phase of the signal. He called it a modified cathode follower with active current sink.

Matt

BluegrassDan · Jan 11, 2018

I use the Hamptone circuit as the basis for my preamp. You should totally remove C8. It is not needed and rolls off too much high frequency. It sounds so much better without it.

I use a Jensen 4:1 output. It’s pricey, but sounds incredible.

mbf90 · Jan 15, 2018

Mine actually does not have C8 because it has stepped attenuators so it is not necessary.

Jensen’s are really nice but also very clean. The Hamptone circuit resembles some of the early 50s RCA pre amps whose sound I love. I decided to further imitate that sound and Use vintage rca tubes, paper in oil coupling capacitors, and carbon composition resistors just like the rca pre’s. I love the vintage sound I’m getting from it. Transformers are next.

GalenH · Dec 15, 2022

BluegrassDan said:
I use the Hamptone circuit as the basis for my preamp. You should totally remove C8. It is not needed and rolls off too much high frequency. It sounds so much better without it.

I use a Jensen 4:1 output. It’s pricey, but sounds incredible.

Hi, do you know of any kits that are currently using this kind of circuit or similar? Total beginner here and interested in making a tube mic pre, ran across steve hampton's article on building one in tape op but his kits are no longer available so I feel like I've found a dead end.

Johnny1234 · Dec 18, 2022

If my math is correct the first gain stage is around 20. The second stage is 34. 3rd stage around unity. Total gain around 680. Gain of 680 is 56 db.
With the 4:1 transformer gain becomes 170 or 44.6 db. Why should I throw away 75% of my gain with a 4:1 transformer, why not a 1:1 or 1:2 transformer.

gyraf · Dec 19, 2022

Why should I throw away 75% of my gain with a 4:1 transformer, why not a 1:1 or 1:2 transformer.

..because it's the only way to get decently-low output impedances from a tube circuit?

rock soderstrom · Dec 19, 2022

Johnny1234 said:
If my math is correct the first gain stage is around 20. The second stage is 34. 3rd stage around unity. Total gain around 680. Gain of 680 is 56 db.
With the 4:1 transformer gain becomes 170 or 44.6 db. Why should I throw away 75% of my gain with a 4:1 transformer, why not a 1:1 or 1:2 transformer.

The WCF output stage cannot drive 600 ohm inputs (e.g. LA2A) without the 4:1 output transformer. Low impedance is the goal. For a modern 10k input on your interface it doesn't matter that much.

BTW, you forgot the input transformer 1:10 (+20db). So there is plenty of gain with the 4:1 OPT as well.

Johnny1234 · Dec 29, 2022

What would be the best HT to use on this pre amp.
Many Thanks
and Happy New Year
Johnny1234

dmp · Dec 29, 2022

If you mean the output transformer with the WCF I used a Edcor WSM in my LA-2A and it sounds great. They are cheap too
15k:600 gives you a 5:1 and 10k:600 gives you 4:1
Either would work - you'll have a little less gain and output Z with the 5:1

Johnny1234 · Dec 29, 2022

dmp said:
If you mean the output transformer with the WCF I used a Edcor WSM in my LA-2A and it sounds great. They are cheap too
15k:600 gives you a 5:1 and 10k:600 gives you 4:1
Either would work - you'll have a little less gain and output Z with the 5:1

No I meant power supply voltage ie high tention.

dmp · Dec 29, 2022

No I meant power supply voltage ie high tension.

Some research to do and then come back with specifics
What B+ voltage?
AC or DC heaters?
Passive PSU or switched (off the shelf)?
Do you want to build or buy?
There are some affordable PSU for tube gear available

Johnny1234 · Dec 29, 2022

dmp said:
Some research to do and then come back with specifics
What B+ voltage?
AC or DC heaters?
Passive PSU or switched (off the shelf)?
Do you want to build or buy?
There are some affordable PSU for tube gear available

AC heaters
Hammond power transformer....that is why I was asking recomended B+ voltage
Building power supply in box with preamps.
Again Many Thanks

dmp · Dec 29, 2022

I misunderstood what you were asking.
The B+ voltage can vary somewhat, I would expect 200v-250v

Or just read the article that schematic came from (I did a reverse image search to find it):
https://tapeop.com/interviews/30/build-tube-mic-pre/
"The high voltage can run realistically anywhere from 150 vdc to 240 vdc (DO NOT EXCEED 250 vdc!), though this design is optimized for 225 vdc."

dmp · Dec 29, 2022

Building power supply in box with preamps.

You might want to think about hum if you are putting a Hammond EI in the box with audio transformers.

Need help with output transformer for tube mic pre

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