Adventures in Laney Land - My GH100L Experience

[ilikehats]

Bought a Laney GH100L locally earlier this year. Fantastic amp. JCM800-ish cold clipper circuit with a tube driven boost on the front end. It was in pretty good shape when I got it. Replaced the tired GT 6L6s with a set of JJ EL34s and it was good to go.

A few weeks ago, she would get very upset with the gain or master above noon.

Oh.

Whatever, its an old amp. Its fixable.

I tested everything and it all looked good. Bias voltage was stable. Plate voltage was right where it was back in April. Screen voltage, normal. Tubes are an Apex matched set and previously biased evenly at about 65% with no issues. I was a little stumped at first. Replaced the screen grid resistors since one of them looked a little iffy. No change.

Checked coupling caps coming off the PI for leakage. Tried a different PI tube. Re-tension tube sockets. Nothing.

I found my notes from the original bias and went back through the process from the beginning and immediately it stuck out. My original resistance measurements of the two sides of the OT primary were 17.5 ohms and 16.0 ohms. They were now measuring 17.3 and **6.8**. The reduced resistance meant that side was biased well above max dissipation. Yep, that will do it. The transformer had developed a short somewhere.

It can't be the OT. Its never the transformer, right? This time, it was. The OT primary resistance shouldn't change. OT was toast.

Ordered a new OT from Amp Parts Direct - a Heyboer APD-8026H 100w OT for a JCM800.

 

[Techdeth]

I love these amps . I prefer them with 6l6s but creamier with EL34s so that’s killer . These are killer amps

 

[Sixstringflannel]

Total sleeper amps. I’ve played this one -affectionately referred to as “Linda” because it’s signed by Linda from the factory. Amazing amp. The work and mods done on it took it to another level. @ilikehats does great work! He helped me rebias my Helios 100 no sweat while we tested out different biases to get it dialed in for my preferences.

 

[ilikehats]

Total sleeper amps. I’ve played this one -affectionately referred to as “Linda” because it’s signed by Linda from the factory. Amazing amp. The work and mods done on it took it to another level. @ilikehats does great work! He helped me rebias my Helios 100 no sweat while we tested out different biases to get it dialed in for my preferences.

My guy, right here. You keep buying them and I'll keep servicing them.

Ah, my dear sweet Linda.

 

[ilikehats]

I love these amps . I prefer them with 6l6s but creamier with EL34s so that’s killer . These are killer amps

Good to hear about the 6L6s. I need to give it another shot with fresh tubes. The groove tubes it came with weren't great.

 

[Techdeth]

Good to hear about the 6L6s. I need to give it another shot with fresh tubes. The groove tubes it came with weren't great.

A guy in my band bought one from the store I worked at and I would set up his rig . He used a sonic maximizer and though it was cool I’d just have boosted it . Also recorded a band who did the same thing ( sonic maximizer ) . So maybe I’m wrong lol

 

[peterc52]

Bought a Laney GH100L locally earlier this year. Fantastic amp. JCM800-ish cold clipper circuit with a tube driven boost on the front end. It was in pretty good shape when I got it. Replaced the tired GT 6L6s with a set of JJ EL34s and it was good to go.
View attachment 418456
View attachment 418457

A few weeks ago, she would get very upset with the gain or master above noon.
View attachment 418458

Oh.

Whatever, its an old amp. Its fixable.

I tested everything and it all looked good. Bias voltage was stable. Plate voltage was right where it was back in April. Screen voltage, normal. Tubes are an Apex matched set and previously biased evenly at about 65% with no issues. I was a little stumped at first. Replaced the screen grid resistors since one of them looked a little iffy. No change.

Checked coupling caps coming off the PI for leakage. Tried a different PI tube. Re-tension tube sockets. Nothing.

I found my notes from the original bias and went back through the process from the beginning and immediately it stuck out. My original resistance measurements of the two sides of the OT primary were 17.5 ohms and 16.0 ohms. They were now measuring 17.3 and **6.8**. The reduced resistance meant that side was biased well above max dissipation. Yep, that will do it. The transformer had developed a short somewhere.

It can't be the OT. Its never the transformer, right? This time, it was. The OT primary resistance shouldn't change. OT was toast.

Ordered a new OT from Amp Parts Direct - a Heyboer APD-8026H 100w OT for a JCM800.

How do you measure the resistance of the OT?

 

[ilikehats]

How do you measure the resistance of the OT?

Simple ohm meter across the OT center tap and the two halves of the winding at the output tube plate. Tubes removed, caps discharged, etc, etc.

 

[ilikehats]

OT arrived and install was mostly uneventful. The mounting pattern was off just enough that new holes needed to be drilled. On first power up after the initial, tubeless checks, she squealed like a banshee. I remembered this from Rob's site when I built my last amp:

If the amp emits a squeal it may be caused by positive feedback in the negative feedback circuit. If your amp has a negative feedback loop and squeals, swap the output transformer wires on the power tube sockets. Swapping the power tube output transformer wires will change the polarity of the negative feedback signal and stop the squeal.​

Swapped the output wires and she was up and running.

I also ordered all new electrolytic caps and a few other small parts that needed attention because "While I'm In There".

New can caps to replace the 30 year old LCRs.

And new F&T filters.

To top it all off, a new set of JJ KT77s. But there, we found more problems.

It was another Apex matched set from Amplified Parts, but this time, they would not bias evenly. 45% on one side, 65% on the other.

The Laney doesn't have bias resistors on the cathodes, I had been measuring bias on each pair and averaging between the two tubes. Usually fine for matched sets, but in this case it was hiding a bad tube. Using a bias probe, I found the culprit. the 4 measured at 21mA, 21mA, 22mA and 43mA. The fun continues.

To confirm it wasn't something else wrong with the amp, I installed the old EL34s and re-biased. Dead even across the board at 31mA. Bad KT77.

Amplified Parts was great, provided a return label and will be sending me a new set.