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ForeWatt Tube SRPP Preamplifier - Support / Update Thread
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Author:  Trackhappy [ 25 Sep 2018, 04:53 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Life is screwing with me now... I picked up this iron, and the Guy promptly tells me he can get another identical one! There goes my Forewatt plans, but what to use now for my Forewatt?? And the Bigger question, what to do with these two Babies... :)

Author:  Diyengineer [ 08 Oct 2018, 19:02 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa


I'm having a repetitive issue with my Forewatt and am not sure exactly what is wrong and what is causing it. I keep having one channel start to motorboat after about fifteen hours or so of use. It happened twice to the left channel and just now started on the right. I believe I have narrowed the issue each time to the LR8 portion circled in the attached schematic.

To narrow it down, I switched the rca Out interconnects and the problem would switch channels so that rules out the power amps. I also switched the 215V outputs from the LR8's going to the plates and the problem switches channels, so that mostly rules out issues after the regulator circuits. Taking a voltage at the regulator out when a channel starts to motorboat, I usually find about 210V instead of the 215V that it was set to.

To fix the left channel the first time I changed out all the parts in the left regulator circuit one at a time and the problem went away. I replaced the trim pots with fixed resistors incase that was the issue. I just a little foggy as to which part actually went bad. The preamp now worked for 10-15 hours or so then the left channel started to motorboat again.

The second time the left channel started to motorboat, I changed out all the parts in both regulator circuits and re-installed the trim pots and the amp worked well for about 30+hours.

Now tonight, when i was listening, I now noticed the right channel speaker cones now starting to motorboat a little. As a quick test, I quickly installed a brand new LR8 and the problem still is there. I did the usual cable swaps and everything points to the right regulator circuit.

I have checked and reflowed all the solder joints in the regulator circuits and aft of them as well.

What could be causing this? Can i rework these portions of the circuits to be improved?

Any help would be greatly appreciated.


Author:  Trackhappy [ 08 Oct 2018, 19:13 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

What are the voltages at the LR8 inputs? Maybe too much and the LR8's shutting down with too much dissipation? Or dropping too low and the LR8 cannot regulate correctly.

Author:  Diyengineer [ 08 Oct 2018, 21:10 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

I’m reading 277V at both regulator inputs....

I just can’t understand why that when I just replaced the right regulator tonight with a new one and trimmed it to exactly 215V that it still is slowly motorboating the speaker. The left channel is also trimmed right at 215V and is fine. And as previously mentioned to test I swapped the LR8 outputs going to the plates and the problem switches channels narrowing the issue to the regulator portion.

Since it isn’t the regulator that leaves the .1 cap, the 470k resistor, the 5k trim pot, and the 4.7uF cap. I can’t imagine these would fail so quickly, three different times. The .1 cap is rated at 160V, the 470k is 1W, the 5k trimmer is 1/2W, and the 4.7uF cap is 630V.

Should the .1 cap be rated higher?

Any help is greatly appreciated!


Author:  Trackhappy [ 08 Oct 2018, 21:57 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

I don't know how to reference a previous comment, so here is a quote from Bruce from further back:
Some of the Forewatts use the LR8N voltage regulators and then the final voltage can be set easily. You still will need to drop the voltage going into the LR8N as they have rather low dissipations and will shut down if the internal circuitry thinks it is too much. This can be rather perplexing when it does as everything looks fine just you get no output. When you turn the power back on it behaves for a short while (can be as much as 10-15 minutes) and then shuts down. It is telling you that the dissipation is too much. Since you need the current, you will have to reduce the input voltage a bit. I have found that they seem to work best with no more than 200milliwatts dissipation, So the voltage across them should be in the 20 volt range at 10ma (each channel uses about that amount - it depends somewhat on the brand of tube used).

At 10mA with 62 volts across that LR8 you have a dissipation of something like 620mW (if I calculated that right). Maybe it is just not keeping up. The suggestion seems to be to change the feed resistor for a higher value until the voltage in is about 20 volts above the voltage out. Why motor-boating instead of shutting down though, not sure.

Author:  Diyengineer [ 09 Oct 2018, 05:13 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Thanks Glenn. I can give that a try.

To note, each of the three times, I’ve never had a channel cut out or shut down from this.

The speaker just starts to slowly pulse in and out and progressively gets worse,


Where did you get the 10mA from?

Author:  stumpy [ 09 Oct 2018, 05:45 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa


I want to build the most universal preamp possible, using 3 valve bases and incorporating a 4s universal, and a modified forewatt in the same case, sharing the same power supply, with a shared stepped attenuator on the output, with a bypass to use the box as a volume control only.

So, the question is, If I run both off a 250v B+, The forewatt will need a 820r cathode resistor for an ecc82. But what if I go to a 1k2 cathode resistor, and use a bigger 10k grid stopper like the 4S does? Will that give the same ecc82/ecc88 compatibility that the 4S has? I would very much like both preamps to be capable of doing the same tube range.

And what about the output capacitors? The 4S uses a 0.47uf. Will the forewatt be okay sharing those capacitors? If I switch the B+ over after the regulator, and obviously always remove the unused valves, it should be okay?

I am planning on using some nice gold plated speaker binding posts to hold the output caps, to enable fast swapping out of capacitors. For the sake of safety, should I add a grounding button to earth them out via a resistor, to ensure they arnt still holding lethal voltages? They would be under a lid obviously, but hopefully no more involved to swap out than changing bottles.

And since people have had trouble with the LR8s, How about if I use a common B+ for both channels, and both amps, using 4 of them in parallel, sharing the same voltage divider, and input and output caps, but each with their own adjust bypass cap, as close to the IC as possible. Obviously recalculate the voltage divider for the cumulative adjustment current.

What do you think? Is this a realistic plan?

Author:  Trackhappy [ 09 Oct 2018, 14:41 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

DIYengineer, the 10mA was from Bruces comments. It would be interesting to measure exact voltage and resistance (820 ohm resistor) and calculate it.

Author:  Diyengineer [ 09 Oct 2018, 14:54 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Ok thanks, I ordered some resistors and LR8K4 regulators in a TO-252 package. They have a much higher power dissipation than the LR8n’s. Even so, I’m going to try a dropping resistor going to each regulator to drop to 235V at the regulator input.

I calculated a 4.2k resistor to drop 277V to 235V at 10mA.

I think I did that right.

I’m still puzzled as to why when I installed a fresh LR8 that the motor boating didn’t go away at least for a little while again.


Author:  Trackhappy [ 10 Oct 2018, 05:32 ]
Post subject:  Re: ForeWatt Tube SRPP Preamplifier - Support / Update Threa

Hmmm. Calculates right but seems too much of a difference from 680. Measure across the 680 ohm (not 820 as I mistakenly said above). Voltage across it when operating, then accurate resistance (may have to disconnect one end for that) and calculate the current. I'd put a 820 in there first and see what it drops to. Even a small change may be enough to see if that is the problem.
As for why, if they are right on the precipice, then strange things can happen. Some time constant in the protection circuit might be just enough to not let it complete shutdown before the current drops and it resets. Who knows. All the best and do let us know what transpires. :)

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