2008 Carlsbro TC60 Repair – Part 2

This thing came out in 2008 it seems:

http://www.performing-musician.com/pm/aug08/articles/carlsbrotc60.htm

Tech Spec

TC60

60W all-valve combo.

Two EL34 output valves.

Four 12AX7 preamp valves.

True class A/B operation.

Twin channel (clean, lead).

1 x 12-inch speaker.

Classic three-band passive EQ.

Series/parallel switchable FX loop.

Accutronics licensed spring reverb.

Extension speaker output.

Dimensions (WDH): 594 x 305 x 500mm.

Weight: 25kg.

 

I got this thing working luckily, for relatively minor trouble. The amp was sold spare/repair but with a “decent speaker” stated in the Ebay listing, but I found it open circuit even though this Celestion G12T-100W looked brand new. This was a real setback after the price I’d paid so I contacted the seller, who is a guitar parts business, and asked for a partial refund of the price of a used replacement at £40.

He was very apologetic and said he had paid a tech to tell him the fault condition of lots of gear, and went by his notes, so he agreed to the refund. He didn’t think the tech was much good – and he was right. This seems to be the main reason this amp didn’t work. I’ll just purchase the same speaker off Ebay now to replace it.

Although I found a duff 8.2 ohm, 7W resistor that connects to the heater pin 4 of one of the 12ax7 valves, this should not have been enough to write off a pretty new looking/lightly used amp, for a dealer.

I got a replacement resistor for 80p from Maplins, re-wired it to fit as it was an axial not an upright type, and re-soldered it in place. This is just a heater hum suppression mod option, like from Blencowe’s pages here:

The resistor connects the ECC83 pins 4 to ground, in yellow below.

I hate these boards for the heat stress aspects of dry joints and generally flimsy construction compared to point to point hand wired amps, because the valve heat rises right up into them when the chassis is mounted upside down – it is asking for trouble on a PCB in my view, due to the expansion and contraction that will occur. With about 16 screws to remove to check the underside for dry joints, tight spade connectors for transformer connections, a power transistor heat sink and ribbon wires to disconnect, they are just hassle.

This one was actually quite easy to check in comparison to the Ashdown FA40 as the control pots are not attached to the main board – that’s another hassle undoing about 12 nuts to get those off to get the board out on those type models.

These PCBs can have chip/zener/voltage triggered relays you can hear clicking in when you power up to certain voltages with a variac too, and transistor/chip reverb tank input and output stages etc. all adding to the unnecessary complexity in a valve amp. I just don’t see the point. There is never an improvement in tone overall compared to tried and tested vintage circuits, and they become less robust with more things to go wrong, and a pain in the arse to test and fix in comparison. Just finding decent A+ and ground probe connection points can be a task without a schematic.

If you want digital channel switching and an LED then buy a stomp box or a modelling amp! The channel switching is push button at the front and a jack socket in back anyway – why do you need digital for that? Supposedly for channel switching noise suppression, but that can be achieved with good analogue design anyway, in Blencowe’s book.

What’s the point of all the chips and extra components (apart from the obvious business model cheaper manufacturing process in a competitive market, for the company – but from the users standpoint)? There is no benefit here overall for me, just the addition of fault risk compared to a much simpler p2p wired valve amp. It is just a 2 channel Push Pull amp when stripped down to the basics, with an external reverb tank, and two triodes saved on an FX send/return – that’s it for all these extra components!

TC60PCB.jpg

For this particular model, there are no digital FX even to justify any of this for me. It may now as well have a good digital reverb for the quality of a small Belton/Accutronics – it’s not a Gibbs after all – though I may fit one now anyway if I can get one cheap, as I loaned this one to a mate for his Marshall Valvestate while I fix this, as his reverb tank transformer is duff.

Anyway – enough moaning – at least it is working and has a nice clean tone, even if the overdrive sounds like a transistor amp…though I haven’t given it a proper sound check yet.

Considering you can get old or even vintage valve amps on Ebay for less than the retail price of this at around £300, I know which I would prefer. Not that it’s a bad amp – it’s great, unless they go wrong like any PCB model – but not compared to the “real thing” for the money in my view. I paid about the same for my vintage TC60 Twin.

As I didn’t have a schematic for this “Chinese” TC60 as I call it, due to the stickers on the transformers, I had to work things out from some main direct connections I knew have to exist for a valve amp, such as direct connection from the rectifier diodes A+ to the output transformer primary etc. to find probe points for measuring circuit DC when doing the first power up tests with the variac. This all went without incident on the first power up tests, to 476V full HT.

One thing I haven’t come across before is the use of PCB screws to create the ground connections for channels 1 and 2 and the circuit overall, as I left out most screws for the test in case there was another problem and I had to remove the PCB again.

Now I know what serious mains hum sounds like when a valve amp is not grounded to the chassis!

Once the 2 ground screws where replaced all noise disappeared leaving a very nice clean tone – comparable to any good valve amp – but not quite in the same league as its vintage namesake – my 70s TC60, or the Klipp or Tremolux. I put the same reference clip of this amp with the others here:

http://stevepedwards.com/wp/?page_id=5486

I’ll do a full sound check at DBS when I get the speaker back in it.

Freq Response:

TC60_2008FR.jpg

I input a 100mVpp, 1kHz sine and measured 40Vpp across the speakers, near to full volume on the clean channel before crazy low frequency oscillation occurred and a nasty low booming, so I couldn’t go to full volume, even through the 110W cab, so I need to look at that at some point to see what’s going on with it. It may have been resonance at volume in the bedroom with the speakers vibrating the benched amp..? I’ll check again when the its all back in its own combo with the 100W Celestion.

I got up to 40Vpp = 20V peak / SQroot 2 = 14.14V RMS into 16 ohm it gives:
P = V x V / R
14.14 x 14.14 / 16 ohm = 200/16 = 12.5W RMS power out, before it freaks out.

The same thing occurs with CH2 so I think it’s a room testing volume issue, but it could be a power tube LFO problem – a bit like speaker flapping motor-boating.
Even with a change of EL34s it still occurs.
This really ODs a speaker – probably why the last one was dead…
Now I think I found the real reason for a spare/repair sale…
There ya go kids – this is why ya don’t buy valve amps with chips in ’em to repair unless you are fully qualified to do so…
I’m not and have no desire to be either. Throw away garbage mostly.
This is not an auto bias amp I don’t think, as there is a bias pot on the board, so that may be out and require setting to stop the power tubes saturating and oscillating…? If’s and maybe’s…
With no schematic I can find it’s a pain also.

This amp discharges from 477V HT to 5V in 10 seconds.