Holland Little Jimi - Power amp Bias Issue

[exocet]

I've had this amp for nearly 2 years but I guess it's around 15 years old. It looks to be very Fender influenced and certainly the power amp stage is standard Blacface with 2 x 6l6.

Over the time that I've had it, the sound quality deteriorated (distortion apparent on trailing end of notes played). Finally it packed up altogether.

I guessed that the PSU caps had failed and so replaced the lot, including the bias supply.

This brought it back to life and for the past few weeks I've only played it for a few hours here and there.

I noticed some more distortion appearing the other day so I decided to check the Bias which I hadn't done since replacing the caps.

The Bias circuit is a slight deviation from BF Fender as the bias adjust only acts on one tube with the other being fixed.

I found that one tube was drawing 40mA with the other only drawing about 5mA.

I checked Plate voltage and both were at 445 volts. Screen Grid was a volt or so lower so all looked ok there.

I then measured the Bias Voltage at the Grid and found that on one tube it was -62 volts and on the other it was - 42. That would appear to explain the low bias current on one tube.

The issue is that the -62 volts is on the fixed bias supply, the adjustable supply is variable within the usual expected range.

Why should I now be getting such a different bias voltage since changing the PSU / Bias caps?

I've tried different tubes which made no difference. I've checked the continuity on the Cathode / Bias sense resistor which also appears to be ok?

The Grid resistor reads 1.5 k on both tubes.

What am I missing?

[martin manning]

It's kind of hard to guess without knowing anything about the bias circuit, but if it's more or less typical there will be a rectifier followed by a cap and two voltage dividers, one being adjustable and one not. The feed to the grid of the tube using the fixed divider will likely come from the junction of two resistors, one of which will go to the rectifier and the other to ground. My guess is that the resistor going to ground is going open or there is a bad solder joint. These are well-regarded amps from what I can tell. It'd be nice to trace out the circuit since there doesn't seem to be any floating around. How in the world did it wind up in Kent?

[exocet]

Thanks Martin, your description of the circuit is spot on. I'll double check the continuity on the tail resistor which I think is standard 27k Fender type value. Sounds like a plausible cause.

I bought the amp 2nd hand, not sure how it ended up in UK as I've never seen a distributor over here that handled them but I am aware of one other Holland amp in the UK.

They do appear to be well made amps, nice big chassis with everything spaced out to minimise induction noise from transformers. The amp has a nice reverb circuit which I cant quite place; again its very Fender influenced but the send and return "interfaces" are different. It also has a variable slope resistor labled "sweep" which I find to be very effective for tuning in EQ for different guitars. The only thing that is not top notch in my opinion is the combo cabinet....the speaker baffle isn't as rigid as I would like and I think that bass response is not all it could be. So I'm in the process of splitting it into a head / cab format. I've had a nice 4 x 10 speaker cab made, next up is the head shell.

[Structo]

I had never heard of those amps.
Sound pretty cool.

Here is some info:

http://web.archive.org/web/200202031801 ... istory.htm

[Firestorm]

I had never heard of those amps.
Sound pretty cool.

Here is some info:

http://web.archive.org/web/200202031801 ... istory.htm

I've seen exactly one of them; chassis was really heavy gauge aircraft aluminum, solid construction, reverb tank mounted in a bag made of indoor/outdoor carpet, had that cool variable slope the OP mentioned. I don't remember the bias supply being setup as described tho, so maybe one of the amps was changed. The circuit takes the reverb send right after the first gain stage (with a dwell control) and returns it to almost the same point (on the other side of the volume pot) so both wet and dry signals get boosted equally by subsequent stages. Makes the reverb sound like it's always in a "big room."

[exocet]

Amp now cooking again, thanks Martin.
The issue was indeed a broken connection to ground although having looked at the layout again I discovered that there was a voltage divider that fed the adjustable bias tube. This had broken free from it's fixing, resoldered and now good.

I don't quite understand how the circuit works because in practice what happens when you adjust the bias control is that it raises one side as it lowers the other. I'll have to draw it out - hopefully it'll make more sense then.

Many thanks

Marc

[martin manning]

Maybe something like this...

[ampdoc1]

I'd be interested in what that slope circuit is and does!!

[Reeltarded]

Variable slope is the key to life.

On Marshalls I run 33k-83k (50k pot) and on Fenders 56k-106k (50k pot) sweeps. You just limit the pot with a resistor off the output and sub that in for the slope resistor. It's the best control on the amp! You use it to push forward or lay back in a mix. Balancing it with the gain control is really the only thing you need to do. I could hardwire the TMB and be totally happy.

[pula58]

I did some wok on a holland lil jimi. I had trouble with some of the ground connections of the tone stack pots. The design relied on the pot contacting the chassis to achieve a few of its grounds. A lug on the pots was bent-back and soldered to the case of the pot, and the case is allegedly grounded by being in contact with the chassis---except that it wasn't!
The Aluminum had oxidized, so, the ground connections were bad. You might be having a problem caused by the same lack of good ground connection.

[pula58]

Another Holland Lil Jimi found its way into my shop.
This one had the same problem: The design/layout relied on the pots case making good contact to the chassis to make the ground reference of the vol (and other) pots.

Once again the aluminum chassis had oxidized suffiently such that the resistance from the pot case to the chassis was around 100k ohms instead of 0 ohms!

So, just in case anyone else out there in ampland has one of these in the shop: Check the grounds!

[Leo_Gnardo]

relied on the pots case making good contact to the chassis to make the ground reference of the vol (and other) pots.

Once again the aluminum chassis had oxidized suffiently such that the resistance from the pot case to the chassis was around 100k ohms instead of 0 ohms!

Seen this in SWR amps and the Seymour Duncan Bass 400 too. Although they made an effort to mask off the area where the pots mount against paint or anodizing, the aluminum develops a resistive layer of Al02 anyway and - there goes your ground. In the SWR's and Duncan, picked up radio too.

[pula58]

Thanks for the info.

I buff the chassis, then, coat with a no-ox compound (i.e., where a good electrical contact to the chassis is needed).

[M Fowler]

Mojotone's Bass amp kit uses a 250kL pot with no other resistors called mid shift but it is just a variable slope control.

I would use 100k pot and 47k set resistor for the slope.