As everyone who’s tried to buy one knows, power transformers for tube projects can be expensive. Even small transformers for small projects. But today I stumbled on a great deal for a small preamp power transformer made by Hammond.
Most people know about Hammond’s 2xx/3xx vacuum tube power transformers. However, less well know is their 6Kxx series which they sell through through major electronics distributers. These are similar to some of the more common series but with better prices and availability.
So today I stumbled on to the 6K88VG by Hammond. The one I found was sold through Allied Electronics. Here’s the part link: https://www.alliedelec.com/product/hammond-manufacturing/6k88vg/70009014/. As of this writing they are priced at $26.51 USD each. I just bought a couple for my parts stash and with tax and shipping (which currently is substantial) they were still just a little over $38 USD each delivered to my door. My shipping was $18 USD but if I’d ordered 4 it would only have been $21 USD. So bulk orders do make it an even better deal.
This has a 500VCT@40mA secondary for B+ and a 6.3V@2A secondary for tube filaments. This is a great little transformer for stereo preamps and related projects. Used with a 6CA4 rectifier this will give a B+ between 250Vdc and 300Vdc and after subtracting the rectifier filament current (i.e. 1A) still leaves enough filament current for three 12Axx7 tubes (300mA each). There are a lot of possible projects for which one could used these transformers.
I just thought I’d let builders know these transformers are available.
I played with these transformers in Duncan’s PS Designer using various common tube rectifiers and even SS. I found it difficult to get less than 315V without using pretty big droppers in the filters or right after. That’s not too high but I was aiming for the 250v suggested for the 4S pre. I might try using one of these 6K transformers, or I might go with a toroidal from Antek. Similar price ($28) and low shipping. This one gives option of 200 or 180V HT at 130mA along with two 6.3V/2A windings. Using PSUD (Duncan’s PS Designer) I get elegant ps using 200V tap and 6CA4, or 180V tap and SS rect., that gives 250-275V B+. So, another low $ option. Antek AS-05T200 50VA.
https://www.antekinc.com/as-05t200-50va-200v-transformer/
Oops. I only ran SS with the Antek toroidal. No tube rectifiers. Sorry for confusion.
I am currently using one of these in a project and high voltage is an issue. First, these are wound for 117V AC on the primary. My mains voltage is a little higher (about 5%) so I get closer to ≈525v. So peak voltage off the rectifier at zero current is about 370v (525v/2 * 1.414). My project draws about 12.3mA total so voltage drop through a 6CA4 is still minimal (about 10V, even with 5x peaks). I could get about 20v of drop with a 6X4, but the hole in the chassis is already cut for a 9 pin socket. So I need about 60v of drop at 12.3mA or about 4.9kΩ. Dissipation is about 0.75W so I need 1W dropping resistors. Looks like another component order is in my future.
Just an FYI, the 4S works very well with B+ voltages up to about 330V. Personally I recommend it to run at a B+ of about 300v these days. Still need a dropper with these transformers, but not as big of one.
Thanks for the revised voltage recommendation. I wasn’t sure just how much higher I dare go and still get the nice performance you experienced. This 4S pre is such a simple elegant design — the reason I like it and have one “under construction”.
Mouser.es have them in Spain which would be great for me for postage BUT, no version with 230V primary. What a shame.
Hawk has them for $15.60 each.
https://www.hawkusa.com/manufacturers/hammond-mfg/6k88vg
$10 for shipping to CO for order of 4.
That’s a great deal! I wish I’d known that 10 hours ago. 🙂
Thanks for letting us know about these transformers, they might even work for some
small (mono) power amplifiers. Great catch, Matt!