This is another vintage classic from John B’s collection for repair. A 1960’s model – it looks like a Blonde with the light colour cloth – with the label still intact – model 6G9B. This amp is in a sorry state due to an incorrect OT amongst other things:
The choke serial number is 1253C3A. This is listed in the box on the schematic for TR2 export model.
The “606 – 2 – 15” is the manufacturer info of the choke – 606 = Schumacher. 15th week of 1962.
I hope to identify its age a bit better with more research. It looks like it IS a Blonde from the label above – 6G9-B. Seems there should be a serial chassis stamp somewhere, but there is too much rust on this chassis.
Tremolux 6G9, 6G9-A, 6G9-B (blonde)
00100 to 01000 – 1961
00900 to 04200 – 1962
04200 to 05900 – 1963
No more Blondes made after 1963? More info from here:
It is a pre –CBS model as the label states Fender Electric Instrument MFG. CO. and from the LE on the label, it was made in May of 1962. This tallies with the choke dates also – April of 1962.
The chassis sticker states 437544 D. Ha! By sheer luck, the photo got the chassis stamp too, that I missed! 03523. Again, confirms 1962, from the date page info above.
This is encouraging:
“The 6G9-B is a great amp and the best of the Tremolux amps IMO.”
Start of Repair
First easy steps – check which valves still work – if any…
On to the first minor problem – the key has snapped off the only present EL34 power valve (of 2 – or 6L6s – see prior Posts on interchangeability of EL34s and 6L6s), so this amp has maybe taken a knock at some point, or just a ham fisted valve removal by a drunken gorilla.
The valve still works though, but I was really careful getting the right pins in the right holes in the Aston. Would NOT like to put a power valve in the wrong pins and break the Aston!
The other 4 X pre amp valves are 12AX7 equivalents and 3 of them blew on testing.
I can’t test the GZ34 rectifier diode as I don’t have an amp that uses one.
Can’t do much more now except research, and component test with the scope to get familiar with this circuit and an idea of the state of components in general. The next major step will be to see if the mains transformer has a 240V winding to use in the UK or not (JB told me it is a 120V model and that is what the label states, but people change things over time), but if not, I still have the Variac for now, which I would use first anyway when I’m at that stage of initial testing. I will disconnect it from the circuit and test it with the Variac first to see if that major component is OK. This amp just “stopped working” while JB was using it last 30 years back.
Seems a lot of folk on that forum above like this amp – a more Marshall sound than expected.
The mains bulb is duff, 6.3V.
Looking at this circuit diagram, I can see the lineage from this Fender to the Marshall JMP circuit now – the long tailed pair before the power section:
De-soldering and testing the PT windings with a 1V FLS test sig gives 2.2V at the secondary that goes to feed the GZ34.
This would be about right for a 120V mains input, as the label states, as the peak at A+ would then be about 120 X 2.2 = 264V then times root 2 = 264V X 1.414 = 374V for A+. On the Tremolux 6G9B schematic, A+ is marked as 365V, so close enough for now to think the PT may be working ok.
Both the 6.3V and GZ34 5V heater windings show about 20mVwhich would be a step down of 50:1, which, for a 120V input would give 2.4V or 5V peak to peak for these windings also. I’m happy at this stage they would work without having to put higher voltages across the mains transformer at this stage and taking a list of readings. This power transformer has 4 other outputs that give about a 1:1 turns ratio, and one wire that gives ½ that. This would be the PT sec centre tap to ground (red/yellow) along with the green/yellow 6.3V heater centre tap, as in this pic I just found on a forum, that shows a good condition lug strip – it is for export voltage options up to UK 240V.
These wires are grounded to a solder blob under the rubber strip.
Now I can put these wires back in the same place, but I can’t assume the transformer can test to 240V (as it is the 117V model amp – if it has the original transformer), just because the lug rubber says so, though the 1.1 times windings gives the impression that’s what they are for.
Also, I can’t assume the colour code windings for this old PT is the same as the picture.
I like to do this disconnected testing for the PT and OT when dealing with an old amp of unknown fault, as it gets you more familiar with its circuit and makes you think about what is going on with it. It gives you time to look over it in more detail also, picking up small things that will need attention, and getting your head back in electronics mode.
This is the bodged replacement OT someone has used to replace the original (which have the same shape and style as the black Fender PT) – not this ugly great block – looks like a Partridge.
The choke has also been repositioned also, as on removal there is the wear shape of it next to its current position. This is duff also as tests open circuit. Getting to be an expensive repair to get it back to original parts – OT sec, valves and a choke already at least…
|Fender Transformer chart|
I need the UK 240V version, not the 68409. Maybe from this schematic info:
If I replaced all the transformers – and with a UK 240V PT, it gets to around 200+ Euros:
A replacement PT retains the original colour codes and dimensions in inches:
TR1: 125P31A https://www.tube-town.net/ttstore/product_info.php/info/p6181_Hammond-291CEX-Fender-Power-125P26A—022723-UPGRADE.html
116,00 EURincl. 19 % Tax excl. Shipping costs
TR2: 125C3A https://www.tube-town.net/ttstore/product_info.php/info/p2145_Hammond-194A-Choke-Fender-125C3A—022707.html
16,50 EURincl. 19 % Tax excl. Shipping costs
TR3: 125A6A https://www.tube-town.net/ttstore/product_info.php/info/p2159_Hammond-1750J-Fender-Outputtransformer-125A6A—022848-Tremolux.html
47,65 EURincl. 19 % Tax excl. Shipping costs
The main smoothing caps are on a PCB mounted externally with the transformers, and 2 of these are 16microF, 450V dry paper caps. All will need replacing probably.
The other oddity that I have read about, but not in detail, is the “ground lift” switch. This would not work in a UK model in the same way as the US schematic suggests, as Earth is a legal safety requirement for the mains plug so would have to be connected at all times.
On this US/EU model, this switch had been connected to the Earth (without a cap) of the mains cable, by someone, not the original cable of course. This is a definite no, no for obvious reasons.
From Blencowe (Grounding PDF and book, p.275:
“…the connection between chassis and mains Earth is NOT broken. The chassis must remain earthed at all times!”
The reason for a ground lift option is to “combat the problem of creating a ground loop via mains earth when connecting two devices together…” – also in the book.
Basically, this switch disconnects the CIRCUIT ground from the chassis if required. I have de-soldered it and left it disconnected as unnecessary. As there are quite a few hard soldered chassis grounds in this amp, it would not work as it should anyway in the UK, because of the permanent Earth.
As I had a spare GZ34, I used it in place of the one that came with the amp, as I don’t trust that, because it also has the key broken off, and no silver getter in the tube. I had already run the disconnected PT up to 120V so happy that windings insulation is ok, so I re-soldered the 5V, 6.3V and HT windings back in circuit.
To test just the rectifier and not involve anything else, I left the choke disconnected which also links to all the smoothing caps, and de-soldered the wire to the 100k resistor that goes to the tremolo valve section. As all the electrolytic caps probably have to be changed, I definitely don’t want those involved just yet.
This means I am only testing this section:
To be able to see if rectification is occurring, the reference point for one oscilloscope channel needs to be at either of the 5V or 6.3V heater windings, with the ground clip on the chassis. This is because this circuit uses full wave rectification, but with only two diodes, so the secondary HT winding is centre-tapped and grounded. This is a tricky circuit to read compared to the other schematics I’ve worked with, as it is easy to lose track of where is more “positive” than ground.
I started to see rectification at about 90V mains RMS AC on the meter, and got the double bump on the scope:
Above – 6.3V heater AC sine reference shape, and 20V X 10 probe = 200V unregulated DC bump.
25/2/14 – AMP revisited
I stopped working on this amp back in the summer as after the 120V rectification test, I got smoke and some watery liquid dripping from the PT. I assumed I had damaged the PT and as I had to research the wiring for this and replace the incorrect OT secondary from this massive Partridge type transformer, and JB not being in a position to buy the parts for this amp, all work came to a halt.
After occasional research on all transformer and chokes authentic replacements from Tube Town:
I decided to look again at getting this thing working, so work has begun again on rechecking the PT for correct mains wiring colours and values to be sure the original is OK or not and may still be used and possibly save 116 EUROS from the above bill when it comes time to order these parts from Germany. This would not be of use at UK 240V still so really it’s not worth doing the work and try save the cost of a new transformer.
As it is such a rare old amp I would only have been happy getting an equivalent spec. Hammond replacement OT and choke to keep the look as well as the component values.
Also, after more amp build and repair knowledge from the Klipp I am better placed to understand this amps circuit as it combines elements of all my prior amp projects.
The tremolo section uses a DC Coupled Cathode Follower design as seen in the Klipp, but works similarly to the transistor tremolo from the Marshall Mercury using the 3 feedback capacitors that phase shift the positive feedback from the 12AX7 inverted anode by up to 3 x 90 degrees again so it causes PFB at the grid to give periodic oscillation.
The cathode bias for the 2nd tremolo triode and for the power tubes cathodes comes from the bias diode that works in a similar way as the one described in the Klipp Post, using half wave rectification from the cap and resistor in parallel. As surmised in the Klipp Post, the negative bias goes beyond the -25V grid to cathode bias value in the Datasheet to prevent red plating when run at high HT voltages.
The negative power tube grid bias voltage at the tremolo Intensity pot comes from the GZ34 winding going negative relative to chassis ground at that windings centre tap. This same Intensity pot point is also used to super-impose the tremolo AC signal (light blue) that modulates the guitar audio signal (orange) at the power tube grid.
This amp is also a 2 channel, 4 input design like the Klipp and Carlsbro, one bright, one normal, each with independent volume, bass and treble knobs, which can be used together. These channels are mixed in phase at the input grid capacitor of the LTP.
NFB is used (blue) in this design, but no presence pot used.
The main issue for me is the original PT transformer which has 7 separate voltage input windings to decipher, with the only way to be sure is measurement via the Variac.
Mains PT Windings Test Results
The first basic test I can do is measure the resistance of each primary relative to the black which was originally connected to the mains neutral, which gives a starting point for soldering them to the rubber connector tags in the correct order. The highest value should be a 240V connection else why have all the taps that should cover 230,220,210,200 and 120V? The highest measured resistance was 8.7 ohms on the DDM. I have a different set of colours than the pic above:
Variac Input Values:
Primary ohms 40V 80V 120V Sec. ratio Reds 40V 80V
Cream 10.3 40V 80V 120 1.9 68 155 233
Brown 9.6 40V 80V 120 2.0 79 160 244
Cream/Blk 9.1 40V 80V 120 2.1 81 165 251
Green/Blk 8.7 40V 80V 120 2.2 86 174 262
Yellow/Blk 8.2 40V 80V 120 2.3 90 183 275
Red/Blk 3.8 40V 80V 120 3.9 150 308 463
All this makes sense for each of the windings as they would relate to the numbers on the rubber tag.
With the GZ34 in circuit, I got asymmetric full wave rectification at 120V AC in again but the transformer is too hot to the touch. Hot smelly liquid is leaking out again. It must have got water damaged at some point. The PT is unusable, so the new parts ordered.
Blk-Blk = 4.7 ohms 240V Primary
Red/red -Red/yel = 74/31 ohms 325V/650V Sec
Yel/yel = =0.4 ohms 5V Sec
Grn/YelGrn/Grn = 0.4/0.3 ohm 6.3V Sec
Red/yel – Red/blu = 4.8 ohms 50V Sec
Bulb 6.3V = 2.7 ohm
I carried on testing as before at the PT stage with the new one and could go up to 240V in slow stages no problem, after creating more suitable ground lug points near the PT for the orange OT chassis ground and two centre taps ground wires and a completely separate mains Earth lug.
Above = 240V unregulated DC 354V with bulb holder cleaned
Above = 240V full wave rectification
After a full day’s work, I got the amp to the point of all valves in place and turned on to 150V mains AC with speaker attached, but got a PFB howl due to reversed NFB OT wires, so shut off pronto and reversed the OT secondary wires at the speaker jacks. That solved the problem, but I now saw extreme red plating of the power tubes at about 180V mains AC. I think this is because there is a separate but insufficient 50V bias winding on the PT secondary that only gives -7V at the Intensity control when it should be more like -36V according to the schematic info:
I have three possible options that I can investigate here that I can see may work?
One is to try a lower value resistor than the 100k at the bias diode, so that more of the 50V goes negative the other side of the diode, as it measures only -7V using the PT sec tap that comes with the new PT. The 100k resistor is dropping about 30V of that available 50V across itself so not enough is available for bias. The diode is dropping about 13V across itself.
The possible second is increase the value of the 33k rectifier resistor so there is a more negative voltage from diode negative to relatively positive ground (0V)?
The third option is to measure the voltage at a GZ34 (red) winding to see if it is sufficiently greater than 50V AC (to make up the 30V DC shortfall required), and attach there as per the original schematic. This hopefully will lower the quiescent DC grid bias to the original -36V as marked on the schematic, so turn down the quiescent DC current through the power valves and stop the red plating.
If calculated for DC from AC, there is about 50V AC currently at the PT bias tap which is about 90V DC. The 100k must be dropping 90-20=70V DC, and there must be about 90-(-7V) = 97V from tap to 33K. About 30V DC more is required, which is about 30/1.4 V AC = 21V AC more required. The voltage at the GZ34 tap needs to be at least 50+21=71V AC to reach the -36V bias.
Also, this implies that halving the 100k that is dropping 70V, to 50k would gain up to 35V which is about what is required. It may not reduce damage current through the diode though…I would have to check the max current spec for the diode if I did that.
I checked against the 763 schematic to find a 1W, 470R here instead – 212 times smaller, so will see what similar value I can find to try – though anything will be an improvement from full red plating at only 180V mains.
A change to a lower value resistor from the 100k to a 10W, 550R I had proved to be an improvement for 1 valve by showing about -50V bias at only 180V AC mains, but one of the EL34s still red plated massively. This shows the difference in valve makes and models.
I changed these mixed EL34’s for another pair – Sovteks.
As this amp has 7 valves in it, I thought buying the higher power PT option would be good to ensure current handling capability feeding all these valves so I have some headroom in the current output of the transformer.
650V RMS is 919V peak DC. Rectified this is about half at around 460V. I’m still seeing more than this at only 200V mains even with all 7 valves in circuit.
Bearing in mind this amp was originally designed to use 6L6s which can’t even handle the higher voltages that EL34’s can I have a way to go to get things in balance. The only reason I’m getting anywhere here is that a pair of Sovtek EL34G+ valves I have can now handle this HT without red plating with -60V bias, at least up to the 200V mains so far, but I’m still above the 450V rail and filter cap limit.
Hmm, maybe some valves aren’t conducting to help drop this value or there is an open circuit somewhere also…?
With no secondary fuse to protect the new PT should a cap fail short it could damage this new PT.
At least I know the Tremolo works as I altered the Intensity control before testing, as its use also affects the DC bias of the power tubes. Modulated mains hum isn’t very musical…
It’s back to checking circuit components and voltages – after a sound check with guitar to get find distorted tone as in the vid:
This had me again suspecting the power tubes bias as being too cold now at -60V so barely able to switch on so distorting the signal. To prove it was this and not a misbehaving preamp triode, I checked the preamp anode voltages finding them at 250V, which is a bit too high when the original schematic has 200V so maybe a dry joint or two as well as the bias issue.
I know from the Maggie experience what a massive affect the bias on the power tubes has at changing circuit wide voltages, so both bias and dry joints could be involved. I re-blobbed all the board joints anyway.
After a hunch that increasing the 550R by a small amount to try to get closer to the -36V at the bias diode schematic value by trying a 5k6 – ten times larger than the 550R I have but only 20 times smaller than the original 100k. The key is not going too high too quick and getting back to a red plating scenario. This was bang on and gave me the extra headroom at the filter cap to get to 240V mains without going too much over the cap rating of 450V DC. Excellent Smithers!
Mains HT Ripple EL34 bias/res Discharge time
220 433 2V -60V/550R distorted o/p
220 420 10V -38V/5k6 no distortion
240 458 14V -40V/5k6 no distortion 458V-10V in 10sec
Here is the sound of a 1962 Tremolux almost as it probably should be:
Also, I realised the bias switch on point acts like a noise gate to the lower volume ripple hum, at least at low volume so far, as this is a lot quieter than I expected this amp to be with all the grounds and loosely twisted heater wires – amazing!
This only leaves some things I want to do like put in a 2nd fuse for the PT in place of the defunct ground switch:
Ah…This is not possible as you need to put two in for a GZ34 at each winding:
I removed the switch anyway and blocked the hole with a nut.
The performance tests such as FR:
voltage readings at 240V:
power output calculation DDM across the speaker:
P = Vsquared/2R
I tested it with a 1kHz sine at full volume and measured 42V RMS output into 16 ohm
42 * 42 / 16 x 2 = 55W
End to end symmetry:
It is a really nice amp and makes my Ibanez sound like a real Strat, on the Bright channel it is unreal!