Hiwatt CP103 build

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shakti
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Re: Hiwatt CP103 build

Post by shakti » Tue Nov 29, 2011 12:57 pm

I've only briefly tried it through a Marshall 4x12. I find it loses so much of the Hiwatt character without the Fane speakers that it's gard to relate to...doesn't have the character or overdrive of a Marshall amp nor the attitude of a Hiwatt, so it ends up falling between the chairs. But I should give it some more spins. The balls and punch of this amp through the SC 4x12 with the very efficient 17000 gauss speakers is phenomenal...nothing else in the world could compete with that...

As for a different OT, it's more out of curiosity. M e r r e n says his Hiwatt clone has a core of a different (and correct) type of steel (grain oriented) which I suspect could translate to a warmer, fatter tone while still retaining the punch.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

jazzy cian
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Re: Hiwatt CP103 build

Post by jazzy cian » Thu Dec 01, 2011 9:18 pm

Sweet baby jesus that wiring is neat! Must be solid core!!!

shakti
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Re: Hiwatt CP103 build

Post by shakti » Sun Jun 26, 2016 6:05 am

I finally got some video clips together of this amp, A/Bing through two different cabs. There are 5 videos in total:



Please excuse all the flubs, this was thrown together very quickly.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

RockinRocket
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Re: Hiwatt CP103 build

Post by RockinRocket » Sun Jun 26, 2016 9:56 am

Very nice build!! Inspiring to try one my self at some point.

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Carbia
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Re: Hiwatt CP103 build

Post by Carbia » Tue Jun 28, 2016 5:27 pm

which were the settings?

That sounds nothing similar to a '73 DR 103 that I used to play (now Neikeel has it)

Well, Now I see the schematics and it has nothing similar with a DR...

shakti
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Re: Hiwatt CP103 build

Post by shakti » Wed Jun 29, 2016 1:43 am

I also have a DR103 and indeed, they do not sound *at all* like each other. The CP103 is tight, sparkly, scooped and chimey, and gains up a bit towards the end. The DR103 is more "neutral" sounding, warmer. The overall gain isn't that different, but the clean-up of the CP103 is the best in any amp I've ever tried.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

Roe
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Re: Hiwatt CP103 build

Post by Roe » Mon Jul 11, 2016 8:40 am

Thanks so much for these clips. Some great townsend tones for sure
http://www.myspace.com/20bonesband" onclick="window.open(this.href);return false;
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Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

shakti
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Re: Hiwatt CP103 build

Post by shakti » Tue Jul 12, 2016 1:59 am

Thanks! There really is no substitute for a CP103 and a proper Fane cab for Townshend tones (pun not intended BTW).
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

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Re: Hiwatt CP103 build

Post by Roe » Thu Jul 14, 2016 5:37 am

Yes, I particularly liked the "H style" cab (17000 gauss speakers?) for Townsend tones. Still, I do tend to prefer his Fender tones, although I do like the Tommy tones quite a lot
http://www.myspace.com/20bonesband" onclick="window.open(this.href);return false;
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Super 100 amps: 1202-119 & 1202-84
JTM45 RS OT JTM50 JMP50 1959/2203/34/39

shakti
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Re: Hiwatt CP103 build

Post by shakti » Sun Jan 06, 2019 5:40 pm

I've been having some serious trouble with this amp lately, and could use some advice in troubleshooting it.

It's been working trouble-free since I built it (well, except a few minor things mentioned below). It has always been a pretty bright sounding amp, on the edge of harsh/brittle, and very picky about guitars. It loves P90s, but a Strat always sounded very boomy and scooped, not a very pleasant tone. But the P90 Townshend sound was what I was after, so I liked it anyway and also never had anything to compare with. ANd another small thing; channels 3 and 4 oscillate when I turn the channel volume up to more than 2/3. They have this high-pitched whistly sound right from the start and a lot of white noise, which turns into wild oscillation when you turn it up. Whatever I tried, I wasn't able to fix it. But the funny thing is; it was NOT there when I first built the amp. But it was another thing I've just lived with, since the 4 channels are near identical and I was only using channel 1 anyway.

Fast forward till a few days ago, and I am now beginning to think there might have been a tiny issue from the start, which has now turned into a big problem. Maybe. Bear with me...

About a week ago, I noticed a low-pitched hum developing from the amp while playing. Looking over, I could see one of the output tubes redplating. OK, no big deal...power down. I had two pairs of Mullard EL34s in there, a mixed bag between xf2s and xf4s and only matched by pairs. But the amp has individual bias adjustment per side, so two pairs are OK. I was thinking perhaps it needed a bias adjustment, or the tubes were beginning to get worn out. But I didn't have time to fix it that day. Next time I went to play it, it worked fine. Then next time, redplating again, and also a slight weird and nasty distortion. Try to adjust the bias, but it's not really off and the tube isn't redplating anymore, but it sounds weird and harsh and distorts right from the start. So I swap all the tubes for known good ones. No change.

Bummer. So this all means that there is a problem with the amp itself, and not the tubes. So we go trouble-shooting...

Is it around V1? No, because it behaves just as badly on all 4 channels (channel 1 and 2 is V1, channel 3 and 4 are V2). These are simply resistively fed in parallell into V3.

So is it around V3 (tone stack)? Could be...but I found an easy way to check; I have a '73 DR103. The signal in a Hiwatt runs out of the tone stack (treble pot wiper) into a simple pre phase inverter master volume. I could simply tap the signal from the CP103's first two gain stages (V1-V3) into the DR103 master volume, and use the phase inverter (and its associated circuitry) plus output stage from the DR103. Result? Amp (well, Franken-amp, to be precise) sounds better than ever. The DR103 output stage is slightly different from the CP103, so it wasn't entirely an apples to apples comparison, but close enough to say that maybe there has been a tiny issue with the CP103 from the start. I am not 100% certain, since the pre-phase invert circuit is a tiny bit different from the CP103, and the negative feedback circuitry is also slightly different (no presence control in the CP103). But overall, this Franken-amp with just the CP103 front end sounded a little bit warmer and darker. Some of the brilliance wasn't quite as good as what I had with the stock CP103, but it sounded much better with a Strat and all the weird stuff was gone.

OK, so now we can conclude that the problem is somewhere between V4 and the output jack.
BTW, I should mention; after the redplating, I checked all the screen grid resistors and the grid swamp resistors on the output tubes, as well as the 1 ohm bias reading resistors.

Now, for another strange finding. When I tried to bias up the replacement quad of EL34s (just a TAD quad I keep around for testing as I know it is a good, matched quad), I noticed something very strange. Even with the same exact negative bias voltage on all four tubes from the matched quad, one pair would always dissipate quite a bit more. Say the left pair would be at 35 mA, the other pair would be up to about 55 mA. But swapping the tubes around, this finding followed the sockets and not the tubes! So one half of the output stage seems to be acting up in some way.

So now I go checking around the phase inverter (and the pre-phase inverter circuitry from the master volume and V4). Redplating and some kind of imbalance in the output stage...hmmm...leaky phase inverter output cap? I replaced the output caps, but no change. Then I replaced all the caps on V4 and V5 except one (a little hard to get to), no change. Also replace the 4,7uF 450V filter cap that feeds V4. No change. I disconnected the negative feedback. No change (except more breakup).
Reflowed all solder connections on V4 and V5. Took readings and compared to the DR103, and they are all ballpark the same.

I am beginning to get lost. Does this ring any bells guys? I'll have to break out the oscilloscope and pretend I understand what I am looking at, but am open to all suggestions. The redplating and the other things I've mentioned may or may not have any relevance; in any case I am left with an amp that has lots and lots of output volume, but sounds like a broken transistor radio. I don't know, but it seems to be getting slightly worse and worse too. Has the OT gone bad this time? Maybe it had a small issue from the start? I am probably paranoid, but at some point during its early life, one of the speaker jacks went bad (one of those terrible fake Cliff jacks which I used unknowingly, they have been nothing but trouble in the amps they inadvertently went into). Thos have of course been replaced, but I may have had the amp running with a bad output jack for a minute or so, I don't know. Didn't seem to make any trouble at the time, I replaced the jacks and everyhing was good.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

shakti
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Re: Hiwatt CP103 build

Post by shakti » Mon Jan 07, 2019 11:33 am

OK, the oscilloscope comes in handy this time. If I'm doing this right, it looks like the signal coming out of the phase inverter is severely imbalanced - the amplitude on one of the .047uF output caps is much greater than on the other side. This imbalance only gets worse the more I turn the amp up (either input volume or master volume). The signals are perfectly out of phase with each other though. The hot signal is...you guessed it...on the side that dissipates much more at idle/needs more negative bias voltage.

Now to find out why this imbalance. Any tips?
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

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neikeel
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Re: Hiwatt CP103 build

Post by neikeel » Mon Jan 07, 2019 1:08 pm

Are you measuring the signal on the grids?

You say you replaced the PI coupling caps (when I've seen this before it was a leaking PI output coupler), reflowed the joints, checked for a broken wire on the bad side, checked the grid stopper is not drifting way up?

When I read your first problem post I was thinking failing OT but forum was down for a while/

Have you tried flipping the grid wires (with NFB removed pro tem)?
Neil

shakti
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Re: Hiwatt CP103 build

Post by shakti » Mon Jan 07, 2019 1:56 pm

Hi Neil,

I measured the signal right at the PI caps, will try right on the grids as well. I will also try to measure with the output tubes removed, so that the OT is essentially completely removed from the circuit.

However, I just remembered that if my memory is correct, the oscilloscope probes have this built in amplification switch. I need to check that they are set the same. I had the same range on the oscilloscope, but if one probe has a higher sensitivity that will of course skew things. I haven't used the scope much at all, so I forgot about that.

I checked all the 22k grid stop resistors. I reflowed all the connections on V4 and V5 sockets.

I have not tried to reverse the grids. Actually the grids (and the OT primaries) are both reversed compared to a stock Hiwatt layout. I had already cut a primary wire for installation when I discovered it was reversed, and it was too short to reach the correct socket, so I had to reverse the PI outputs accordingly. But that shouldn't make a difference.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

shakti
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Re: Hiwatt CP103 build

Post by shakti » Mon Jan 07, 2019 2:24 pm

Ahem...OK, so one probe had a 10x sensitivity. When they are set the same, the output from the PI looks the same on both sides (only OOP of course). So I am still in the dark here.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

shakti
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Re: Hiwatt CP103 build

Post by shakti » Mon Jan 07, 2019 4:53 pm

However, even if the amplitude of the signal coming out of both sides of the PI is the same, there does seem to be some kind of weird imbalance in the output stage. If I take live readings of the dissipation while playing (measuring across the 1 ohm bias reading resistor), one pair of output tubes dissipates much, much less than the other. And again, it's the pair that needs more negative bias voltage that actually dissipates more, shooting up to 200 mA+ on the peaks, whereas the other pair barely shoots past 60 mA on one tube and occasionally to 100mA on the other. And sure enough, the coler pair remains much colder, can easily touch it with my fingers even after 15 mins of high volume playing, while the other pair gets plenty hot, waay to hot to touch.

Measuring on the primary side, the DC resistance for both sides of the OT looks about right though, about 14 ohms on one side and 16 on the other.

I am somewhat on the track of it, but still I am lost at where to go next. I think I need to properly evaluate the OT.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

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