Anyone heard of Hilgen amplifiers?

[mort]

A B2501 Basso model should be hitting my front door tomorrow for repair. I was looking over the schematic for it and it looks a bit different from the typical Fender type stuff that I've worked on in the past.

First question, what are those bottom PT taps in the schem? What kind of little circuit are they feeding there??

I'm sure I'll have some more questions as I dig into this thing. Apparently it hasn't sounded right in a long time and there was some shotty work done on it at some point and a replaced OT. I'll post some photos when I get it.

[Enzo]

What taps you mean? I see secondary windings from top to bottom:
high voltage
5v rectifier heater
6v tube heaters
bias supply.

This is an fairly common looking amp circuit. B+ supply is generic. The bias supply is a little unusual in that it is drawn up in the amplifier area instead of the power supply are, and the voltage divider is on its AC side instead of the DC side. But those are small details. I see no center tap or other ground reference for the tube heaters, and the amp ought to have one. The power amp itself has a bit of cathode bias as well as some fixed bias.

[mort]

OK so it must be a bias supply. Because of the location of where it enters the circuit and the location of the diode I thought it was for some other purpose. Thanks for straightening me out.


Question #2

Can someone explain what is happening at the tail end of each channel, just before they come together? The channels are identical up to this point. My understanding of preamp circuits is very very limited.

[Enzo]

The two tube circuits look the same, the lower one has a brightness cap on the volume control and the tone control is a little different. The lower channel will be brighter.

[mort]

about that brightness cap, does the signal get brighter as the value of the cap goes up?

[Enzo]

No, the larger that cap value, the lower into the audio freq range the effect goes. In other words a tiny cap would affect only the highest freqs - the tweeterest of tweeters, while a larger cap will start to include some upper midrange as well.

[mort]

oh wow I got this thing in the mail today and here are some problems observed without even plugging the thing in...


- last filter cap replacement installed backwards and wrong value.

-lotta dust and hair in there. Needs bath badly.

-OT connections are wonky. One lead appears to have it's plastic sheath melted *into* the solder joint.

-some resistors/caps have been pushed against and their leads bent into each other causing shorts. (true ptp)

-there are some huge solder wads in there. not really a problem electrically but should be tidied up

-OT secondary runs into a 'floating' or 'dangling' speaker jack. Typical chassis mounted jack needs to be installed.

- little cathode bypass caps for the 12ax7's spilling their guts.

[mort]

hey on that schematic in the first post, I have a question about the resistor before the bias pot(R20). Is that more than likely supposed to be a 4.7k resistor?

The one in the amp is a 4.7k but it's been monkeyed with alot and hasn't been working right in years.

[Enzo]

Maybe. The test of it is does it work? Is the range of bias voltage appropriate? If so, leave it be. Yes, a 2/1 voltage divider there makes a lot more sense than a 10/1.

[g1]

4.7K is probably correct. 47K would probably drop too much AC from the bias winding, resulting in too little bias voltage, and tubes running very hot, possibly red-plating.
Are you getting a reasonable range of bias voltage at the grids of the power tubes?

Also, you mentioned a "backwards" filter cap. Note that the bias filter cap is correctly installed when "backwards", with positive end always going to ground (in this amp C11).

Edit: That's an odd bias trimmer, working as a shunt on the AC side of the bias rectifier. I don't think I've seen that before. Probably could use a series resistor so that bias can't be turned off.

[mort]

I don't have a power cord in it right now and it's half apart, undergoing major surgery. I'll just leave the resistor for now and see what I get when it's all back together.

[mort]

Is there enough info in the schematic to determine what a good OT replacement would be for this amp?


I'm just curious because it doesn't have an original. What's in it appears to be a cheap Deluxe Reverb replacement tranny.

[g1]

Is there enough info in the schematic to determine what a good OT replacement would be for this amp?

No. At minimum, power tube voltages are needed to determine what kind of power capability the OT needs. And speaker impedance.

[quayhog]

I apologize for adding to a log dead thread but maybe I can add to the discussion by restarting it. I've acquired a B2501 as described by the original poster. I need to change the power cord and recap it. There have been a few ham handed "techs" in it during its lifetime. I need to order a multi-cap from Antique Electronics supply. It a vault they have. This needs to be done before I dare start it up. It's presently not safe to run.

It has some ARS marked JJ 7591a tubes. My history with JJ's has been very spotty. I have several NOS posibilities in my caddies. I've only used them in kathode biased amps. This variation has an adjustable fixed bias mod. I've seen Hilgen schematics that are kathode biased. My question to techies here is, what is the best milliamp setting for the 7591a. Both NOS and the JJs. I'm sorry I don't have the plate voltage yet. I assume it's around 420 vdc. The caps are 450 volt.

It has the original speaker but it has a 50 cent sized hole in the cone. I have a spare JBL D-140 and a reconed D-130. I'd love to use the amp for my acoustic bass (standup and bass guitar) with piezo pickups. Anyone here have a simple mod to maximize the potential of piezo equipped instruments. This would be low volume recording use. I'd love some suggestions.

The amp is not in good enough cosmetic shape to worry about collector value. It's an interesting circuit.

Jonathan

[MarkusBass]

I'd love to use the amp for my acoustic bass (standup and bass guitar) with piezo pickups. Anyone here have a simple mod to maximize the potential of piezo equipped instruments. This would be low volume recording use. I'd love some suggestions.

If the piezo pickup is passive, you may try to increase the 1M resistor on the input to let's say 3.3M. This will improve the frequency response of the pickup.
Can you post some photos of the amp?

Mark