my first dumble build

[dragonbat13]

I want to build an ods style amp. I am thinking of a D'lite Here is what I have for iron.

Came out of a hammond tone cab. I have the OT but have no specs.

I know that the PT has 347-0-347 v. It has 5v and 6.3 volt windings as well. The tubes used were three 12ax7 and two 7591 power tubes.

Would this be some good output tubes and iron for a D'lite?

[Funkalicousgroove]

a 347-0-347 PT will give you DC around 440vDC under load, as long as you use JJ 6V6's, or any 6L6GC's you should be fine.

[Luthierwnc]

I have a donor chassis from a Hammond 100E. It also used two 7591's. I tested the OT winds and got a primary Z of 6150 at an 8 ohm secondary. Big piece of iron, at least in my case. FWIW, that primary puts you closer to 6L6's than 6V6's.

If you want to stay right on spec, JJ also makes a nice 7591 that is a good compromise between the 6#6's. I'm not sure how different it is than their 6V6 but I've gotten a little more headroom and bottom out of it.

Forgive me if you already know how to check but you run wall voltage (a variac is better but it should be fused and connections insulated) into the outer primary legs and measure the secondary voltage. Divide the former voltage by the latter to get the turns ratio. Square that for the impedance ratio. That gives you how many z's in to out. Calculate either what tubes you want to use or what speakers you want to use to figure out what the other component is "looking for"

Skip

[tele_player]

Won't running wall voltage across the primaries blow a fuse or trip a breaker? Is the resistance of the primaries enough to limit the current? Should this test be done quickly? Should it be done with a dummy load on the secondaries?

A variac sounds safer to me.

[Luthierwnc]

You're putting 450VDC into those wires as an amp compared to just 120 VAC during the test. In this case, I put exactly 100 VAC into the plate legs and got 3.607 volts out. 3.607 into 100 is 27.726, times itself is 768 per output ohm. I've done it lots of times and never blown anything up. I suppose you could use a lesser input voltage but the bigger the primary, the more accurate the secondary reading. Just make sure you tape off the connections or use wire nuts. Dial in the variac first.

One thing people are re-finding out now is that the impedance under load is a lot different than the static tests. Plus, a lot of those older tube spec charts were nowhere near the kind of volts in actual use. Try to get 425 VDC stats on a 6V6.

One last thought, Hammond output transformers for 6V6's from that era are something like 10k Z. I've got one of those too.

sh

[tele_player]

I was concerned about current more than voltage. You say it doesn't blow up, so the primary's wire resistance must be enough to keep the current down. Good enough for me.

I'd use a load resistor on the secondary.

[dragonbat13]

My variac has a voltage and current meter. I will test mine and let everyone know what current readings I will get. I will post on a new thread.