Early Herzog help - transformers

[traynorbassmate]

Ahoy,

I'm trying to copy a friend's Serial 38xxx RB's herzog. It does not match up with the schematics.

I've got everything down except i'm unsure of which type of transformers to source. I know this is similar to a champ, but what I hold before me is a bit of a puzzle.

There are 2 power section transformers:

Both are labeled E84506.

Both have 2 wires on the primary and secondaries (no center taps). The "first" (which has line AC voltage on the primary) spits out 6V through the secondary. The second transformer taps the 6V output from the first trany for its primary. The secondary connects to the rectifier and electrolytic caps.

Both transformers have red and black wire pairs, except the first uses Red for the primary and Black for the secondary, although the second transformer taps the 6V secondary with Black, and has Red connected to the rectifier.

What is going on here?

Secondly, how do I figure out what type of output transformer I need? It also has only 2 wires on both the primary and secondary, no center taps. Do I simply need a transformer that should expect the same input voltage and has an 8ohm secondary?

Thanks!
Sorry for the noob questions.

TBM.

[martin manning]

Sounds like the power transformers are two filament transformers wired back-to-back, so you get the 6VAC for the filaments and an isolated AC voltage at mains level after the second one for the high voltage supply. This is a common solution employed when mains voltage is sufficient for the circuit.

For the output transformer, you can use one sold as a replacement for a Fender Champ. It will have an impedance ratio to match a 6V6 (I assume that is the power tube in this Herzog) to an 8-ohm load, and be constructed for a single-ended amp of that type.

If you've drawn up a schematic, you might post it here for amusement.

[The New Steve H]

I totally KNEW those schematics were crap!

*&R%(@&%@!!!!!!

I'm going to gut my Herzog and start from scratch.

[selloutrr]

Make sure the champ transformer you buy is 8ohm.
by stock the champ is a 4ohm OT.

[The New Steve H]

I used a Classictone Champ transformer with 4 and 8 ohm taps. They sound different. Adds some variety, I guess.

[traynorbassmate]

Hey Martin - thanks for the info. I suppose this reduces the costs associated with building the amp as I only need to find rather simple filament transformers 120V > 6.3V. Where is the cheapest place to get transformers? (both power and output. I've been looking at Digikey and Mouser).

Is there way to double check what exactly the transformer does? Do I measure the impedance on each pair of wires? or do I just measure what comes out the other end when 120V is applied?

Steve, FYI my model has a relay for the effect on/off, but is definitely a RB's HERZOG. On wednesday i'm going to get yet another same era model to compare notes on.

Shall we source parts together? what do you use for the chassis and faceplate?

FUN!

[The New Steve H]

You don't want to use the kind of chassis I used! I've been experimenting with aluminum tubing and aluminum channel, and people gave me hell for it.

I'm trying to motivate myself to build a wooden cabinet for the Herzog. I plan to rout a rectangular hole in front side for the knobs, and I would use waterslide decals on the hardwood below them. It's pretty much impossible to label aluminum nicely, unless you want to stuff a 100-lb engraving machine into your workshop or spend big money on legend plates.

[traynorbassmate]

Any issues I should be aware of when adding center taps to the circuit?

I've measured the input on the rectifier and the 2nd transformer is indeed the same but reversed from the 1st one. 116VAC! Why do I need a champ replacement then, which can pump out 300+ VAC?

My drawing skills are very very crude, my first attempt at TinyCad.
Check out what I have so far.


As for the output transformer, does it help at all to measure each winding with a DVM meter? I get 2ohms on the output and 160ohms on the primary. Or do I have to measure with 12VAC going through?

[camerongrieve]

Someone correct me if I'm wrong, but I'll try and help you out...

The Champ replacement transformer mentioned was in regards to the output transformer. This is probably the most critical component of the build to get right. Measuring your OT with a DVM won't yield much useful information. The important factor is the impedance, which is different to the resistance, even though both are measured in Ohms. The OT you're after will have a primary to secondary impedance ratio of around 5K : 8 Ohms.

Also, the diode and capacitor arrangement hanging off the second transformer in that diagram is a voltage doubler, which will give you, I think, (2 x Vpeak) volts DC, or 325V, give or take.

Hope this helps

[traynorbassmate]

Thank you, things are definitely getting clearer.

So since I don't want to start changing up the rectifier maybe I'll just stick with buying two filament transformers.

How many amps should these be rated for? Does it matter if I get an open body transformer instead of an enclosed version?

6.3V @ 4amps enclosed transformer is $37.

6.3V @ 4amps open body transformer is half the cost at $18.


Back To OT:
Since the Champ replacements are all 4ohms, how does a Blues Jr. OT sound? 15W, 7000ohm PT, 8ohm sec. Cost: $22.
How will a 5K v.s. 7K PT change the sound of this project?

Thanks again!

[martin manning]

Since this thing is going to be used at the front end of the signal chain, I'd go for the enclosed transformer to minimize the possibility of hum. I'm guessing that's what the original has? You could also use a Champ PT, which you can get for ~$60. Here's a stand-up mount version: http://www.triodestore.com/5f2potr3270m.html Re the OT, you need a transformer designed for a single-ended amp. The core is constructed with an air gap to avoid saturation. Since a Blues Jr. is push-pull, it wouldn't be the best choice. There is a Champ type with 4 and 8-ohm taps at the link above.

[traynorbassmate]

So I have another Herzog here, 1 thousand off the serial number of the first model I used. This one only has one power transformer and a mult-section FP type cap. Details to follow.

One of my intentions with this preamp is to use it as a Leslie preamp. However, I tried the herzog today and it is nothing but drive!! How would I introduce a clean channel?

[traynorbassmate]

Do I up the input volume - say to 1M?

[diagrammatiks]

buy the low voltage filament transformer from edcorusa

they just released an entire line of low voltage transformers topping out at 15-0-15.

20-30 dollars.

[traynorbassmate]

From what I can tell they are all open style designs and it was recommended that I need closed housings to eliminate the chance of introducing hum.