Carlsbro CS60 Reverb Mk II PA Repair

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I am going to just add stuff like vids and schematics to this Post over time maybe, as there was no need to do a real in depth check of this amp before getting it going, as the bay seller had fired it up, got no sound then sold it as a spare/repair:
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ORIGINAL 1970s CARLSBRO CS60 REVERB Mk II PA AMPLIFIER
NOT WORKING – I am selling this for spares or possible repair.
All parts and valves are present – see photos..
I have tested it by plugging in to mains and connecting a speaker. The three valves on the right of the rear photo still glow, the two on the left do not. The ‘power’ light comes on at the front. No sound.
The ‘smartie’ buttons are missing from several of the knobs.
The amp has been stored in a garage lockup for several years.
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It wasn’t too much of a risk to think it was probably just a dead valve – which turned out to be the case – so a real bargain for an absolutely cracking amp in top condition!
After the usual cleaning the chassis with a paint brush and white spirit to remove that fine greasy black residue that all these old amps seem to attract, and a bit of rust, then spraying the inside with IPA to remove dust and cobwebs, and the case cleaned with a nail brush, soapy water then Mr Muscle and Pledge to get rid of the awkward stains, it came up real good:
before
PreClean.jpg
after
Front.jpg
DSC_0001.jpg
DSC_0002.jpg
Look familiar to anything…?!! Like a JMP Marshall for instance..?
My usual test procedure – before turning on any unknown amp is something like:
1 Test the valves in the Aston kit amp.
It can be good news to have one failure, as this is probably the only reason for a non techie owner selling as a spare/repair bargain…
This dead valve was a bit unusual though – mmm..pretty colour!

2 Check the plug Earth to chassis connection – 0.5 ohms or less – SAFETY FIRST!!
3 Check all the caps with the scope component tester – get an ellipse for all but the smallest values.
Check the big filter electrolytics with cap tester for actual values. If way off then probably failed. Look for bubbles/leaks and general condition. (All this won’t guarantee against an old cap blowing under full HT of course.)
4 Check the rectifier diodes with DDM to check 0.5 ohm forward conduction and open circuit reverse bias:
DSC_0013.jpg
5 Maybe check all/some resistors with scope/DDM depending on condition of amp overall or burned/tired looking specimens while referring to schematic:
60par.jpg
for those in parallel that may give false impression readings.
6 Check continuity/resistance of PT and mains switch function using DDM
7 Check continuity/resistance of choke and OT windings.
8 If happy enough overall, mains test with valves removed, checking diode A+ voltage (510V), and ripple (7V) on scope which shows rectification occurring:
DSC_0014.jpg
DSC_0015.jpg
9 If no smoke, fires, bangs etc. power off, timing the discharge time in seconds (I can hear my wall clock doing this while writing down a voltage every 5 secs if I want to do a graph in Excel later) so you know how long it takes for amp to become safe to touch (e.g 510V-20V in 20 seconds)
10 Replace valves and redo step 8 with all knobs at zero and speaker connected. (A+ 503V, ripple 15V)
11 Increase volume gently until mains hum heard. Poweroff.
12 Setup FLS test signal at 200mV on scope for a powerful amp like this. Plug FLS into an input, make sure volumes are down. Connect test probe to speaker out jack for testing end to end test signal distortion.
13 Power up again, increase volume of input and main volume gently until sine wave is heard. Check input sine shape against output shape for symmetry, and phase relationship (should be in phase). Poweroff.
14 Do any further tests you wish, or test with a guitar for tone and all control functions – crackly pots, presence, bass, mid, volume etc, all input and output jacks, reverb tank (if connected).
15 Re-assemble, test and take to studio for a mega blast through a 100W cab, with rest of your kit, and video it! (tomorrow! Yay!)…
29/11/13
The only other thing wrong with this amp is the reverb – as I wanted the amp useable for the Thursday jam, I didn’t try this til all was back in the case, as the reverb tank is fitted to the inside front of it – a rather large one too:
RevTank.jpg
It rattles a lot this thing, but my first guess on a fault would be one (all?) of three transistors for the reverb send/rtn circuit from and back to the first pre amp valve – has failed. This section is on the right of the main board:
CS60board.jpg
ReverbTrans.jpg
Another odd thing is the choke and OT slightly angled, like they had a knock – but both the same angle – the only thing I can think is that the amp was left standing for years on end and their weight had caused them to bend their mount points – weird..
WonkyTrans.jpg
The amp sounds great at volume, and as with all my other amps with the Guv2 through it, it enhances the harmonics but still has an individuality with the same FX setup I use.
The most amazing thing is there is practically NO hum on full volume, just the same hiss you get with any amp at full volume and no signal – amazing.
The other thing was there was NO ripple immediately after the 10 Henry choke when I checked yesterday – no pic yet though – so I’m wondering if this is a much better filter mechanism that was dropped in most later designs just for cost and weight reasons? I wish I had checked the JMP Marshall for this now too, as that was also quite quiet hum wise for a 100W amp…
I got a lot of Smartfone vids tonight – amazing leap in HD quality now – just gotta edit something to show the amp sound…
So – as far as build quality and components go, John Chambers at
http://www.chambonino.com/
is right – these 1970s Carlsbro valve models are as good as Marshall and Fender amps of the day – just never got the recognition they deserve. It was only looking at the schematic for the 100W model on his site, I realised it was practically the same as the JMP 100W.
I’ll still have to shrink the vid size a bit in HD, but here’s the amp with Echohead, Flanger then clean. Note any distortion is either the speaker at the 60W limit, or the fone mic, and any hum is due to my cheap FX PSU (not designed/filtered for audio).


Reverb Repair
This turned out to be a bit laboured for what was basically a corrosion issue, and the unnecessary cutting of a ground wire, from what I saw…hence the comment on the vid to come – “strange things people do to amps…”
I originally thought it may be the reverb transistor stage,
60parRevStage.jpg
but the more I thought and the general condition of this amp with all else working fine, it seemed less likely, so I got the tank from the case and saw a bit of oxidation at the phono contacts, so thought I would get some numbers for the tiny transformers at each end.
rev_0001.jpg
rev_0007.jpg
rev_0005.jpg
rev_0004.jpg
This was interesting, as I didn’t know exactly how the input and output impedances may differ in a unit – turns out in this unit they are about the same, so this implied that it wouldn’t matter which way round they were connected either, right? I checked this in the Fallen Angel and it made no difference.
Ok, so now the fault pointed back to the to CS60 – wiring, contacts or transistors…? On testing the ground contacts on the reverb tank, they were open circuit between each other, so it looks like the holders that are riveted to the tank chassis are not a good contact. This is where I looked at the schematic to notice the ground on the send side is missing:
60parRevGrd.jpg
As the ground is common to both send and return, it seemed odd that this schematic implies that the comon ground is via the reverb chassis and braid of only one lead. When I checked the amp section, this ground had been cut as per the schematic, but I don’t believe this was done in the factory, but after:
rev_0003
When I attached the tank cables back on the CS60, there was a really big 150 ohm resistance between the phono grounds again, which when dealing with a probable line level send signal of 1V or less maybe, it may make the difference between no signal being received at the tank, and should corrosion set in at the phono push contact as it seems, there will be no signal.
There is a 2k2 resistor that looks like the obvious original ground connection point for the cable braid, which is indeed connected to the 47k next to it so I stripped and reconnected the cable braid there:
rev_0002.jpg
Now the send signal does not rely on the ground connection via the tank chassis and return braid to have a ground of its own..and guess what? It now works…

With the guitar, it sounds lovely. This is probably the nicest and quietest sounding valve amp I have played through yet. I’m SOOOO pleased I got this going! I want reverb like this on all my amps in future!

Oh yeah, the foot switch jack socket on/off works too!
If you wan to know more about reverbs:
http://en.wikipedia.org/wiki/Spring_reverb#Spring_reverberators
I am going to check the Benton unit from the Fallen Angel tomorrow as well as the other no name unit I have, with this amp to see how they sound against this Gibbs. Theory would suggest the larger the better (longer springs) – more bass reponse, richer tone, smoother reverb tail etc…?
30/11/13
Here’s the Benton in comparison:
Benton.jpg
This measures only 12 ohms at the red input and 7 ohms at the white output…I wont send full reverb level to this one as it was not designed for this CS60 for in/out impedance.
See Blencowe Chapter 12 page 210 on FX Loops for more info on TX/RX impedance.

The Ebay no name gave unsettled readings that bounced around from 50-90 ohms so…
NoNameTank.jpg

Seems the larger (longer springs) theory holds true for quality tone in reverbs as expected…again – glad I took the time to experiment for myself – interesting. How much do Gibbs reverbs cost on Ebay…?
Looks like $50 dollars or so for mine.
http://www.ebay.co.uk/itm/1966-Hammond-Gibbs-Reverb-Tank-1122-6749-GOOD-vintage-GUITAR-ORGAN-pa-/190854146260?pt=LH_DefaultDomain_0&hash=item2c6fcb28d4
For details on the function of spring reverbs, look here:
http://www.valvewizard.co.uk/reverbdriver.html
http://sound.westhost.com/articles/reverb.htm
Here’s an FR curve for low volume (3) across the Marshall Marquee 16 ohm speaker:
CS60FreqResp.jpg
1/12/13
Not much left I want to do except take some ref voltages from this amp now, but as a bit of a 1 off, as only old amps have the Bulgin connectors now, I thought I’d show it. Its wired the same as current UK plugs as viewed from the rear, with the key opposite the Earth:
Bulgin.jpg
Volts decay/time (secs)
VoltsDecayTime.jpg
I took some phase readings, DC ref quiescent voltages for future reference around the LTP stage, and NFB ac ref voltages which I have put on the schematic below.
60parVolts.jpg
There are a couple of different things I want to log with this amp I haven’t come across before also. Firstly, the speaker output jack is out of phase with the input! I don’t know what that’s about as my understanding is that any amp should always output a signal in the same phase as what goes in…
I had to double check this as I saw this when I first got the amp but ignored it then as i wanted to get it playable.
As I wanted to investigate the NFB link today, I had to come back to it, and went through the amp stages with the scope to check phase at each stage to see if it makes sense with how the valves are wired – which unfortunately it does!
I read out of phase at the OT secondary again. As this links back via a 1k NFB resistor to the control grid of the long tail pair (also out of phase – so correct NFB as the voltages are the same phase so oppose), it seems you can look at this as the NFB is applied to the control grid of the IN PHASE half (pin 7) of the lower LTP triode. The way I am seeing this is you cannot add NFB (or PFB) to a LTP stage cathode because one half cycle would get NFB only while the other got PFB only for its half cycle, but I may be wrong in that. (Yep!)
If you add NFB to one side or the other though, you should get a gain reduction in both sides as they work in tandem, as a reduction of one side will automatically limit the other side. This would correspond with what the readings I took also below but I am just theorising how this works by drawing on the example from the Maggie values from the Handbook amp Post:
“I measured 30mV at the cathode for a particular volume pot setting, with a corresponding 80mV at the OT secondary. Putting these in the dB equation I get:
20 X log (80/30) = 20 X log 2.666 = 20 X 0.425 = 8.5dB NFB is in operation.
This seems a perfect compromise between 6-12dB recommended.”

The numbers for a 1V ac output at the speaker jack are cathodes (pins 3,8) = 180mV and grid (pin 7) = 60mV
for the out of phase half of the long tail pair.
This seems to work by reducing the grid voltage compared to the cathode instead of increasing the cathode compared to the grid in the Maggie example, which is of course the same thing:
Putting these in the above dB equation I get:
20 X log (180/60) = 20 X log 3 = 20 X 0.477 = 9.54dB NFB is in operation.
Still between 6-12dB recommended by Blencowe.
There is one operation I am still unclear about with this design, which is the 5th diode that returns to the top of the mains secondary via the 180k resistor. It’s connected to what looks like a 100k trim pot but unclear in the diagram. It may be the power tube bias adjustment…more research required. This pot isn’t present on this amp though, unless its on the back of the board?
No, it isn’t – here’s a pic from Chamber’s site:
carl3g.jpg
PowerCathDiode.jpg
Also there are only 3 transistors not 4 in the reverb section, I guess one larger one instead of the Darlington pair?
Finally, one last nice touch not seen yet in any other amp, is Blencowe’s single ground point for the whole amp – even though not separate from a chassis safety earth which is now standard.
Grd.jpg