Nolan 50w Multi Purpose Amp Repair

[Rontone]

I picked up a broken Nolan amp as it was local, after having a research online they are branded Flame in the US, Pat Nolan was an ex Selmer employee, it appears inside to be a Fender type circuit, with EL34 power tubes, but the tone-stack is a bit different,

I've got it cleaned up to check the damage, the main problem is the mains TX primary has blown, open load on the meter, secondary seems fine, the 150 mA HT Secondary fuse is fine, original brand to the holder too, as is the primary 1A fuse, but that doesn't mean they haven't been replaced in a previous repair attempt...

It has a cracked fuse socket, so I replaced the pair, one octal socket had a tiny crack so found a matching part to have an original look, and got fresh F&T caps to put in, cathode bypass and all other caps to refresh, new belton sockets to replace the rusty preamp sockets

Plate load resistors say 100k but have drifted up somewhat, all around the 140k region, so will be putting fresh metal films in, the PCB board is nice, component removal is easy, and some spare eyelet spaces are available to separate the shared cathodes of V1B and V2B

It needs a main power TX, I was thinking the Modulus UK Trainwreck type transformer, 300mA 300v-0-300v CT HT with plenty of 6.3v heater current at 7.5A,  they do a custom wind, so I could get it without the 5V rect as this will be 1N4007 rectifier, [or this could be used for a relay?] I could get a bias tap, or just steal it from the HT,

The bias circuit is ready to be altered and a bias-pot added to the rear, also plans were to keep one channel stock and see how leaky the preamp/tone-stack caps are on first test, alter the other channel by adding a bright switch, mids pot, FMV tonestack, so possibly another valve for a gain stage/cathode follower to make one channel quite 'Marshally' and then the original channel stock Fender type preamp and its original tonestack with possibly a 3 way switch on the mids resistor, maybe a valve driven effects loop too as I use a tape echo, so 5 preamp valves altogether,

I am researching the current draw calculations to get my maths right for the power section, to see what dropping resistors would work best, also to consider adding another RC filtering node if I were to add 2 extra valves,

The 300VAC Should give me 420VDC ish after the rectifier for the EL34's, would this be sufficient to get this beast back to its former glory? It had a 150mA secondary fuse so I presume current draw was under that before,

If I add 2 extra 12AX7's, and were to adjust EL34's bias, which may alter the current draw, a 300mA rating transformer should be well above specs?

[Rontone]

A few pics of the beast

[Rontone]

A fistful more...

[Rontone]

When calculating current draw, I have seen examples saying 3mA from 12AX7/ECC83 types, is that per triode?

Does an entire 12AX7 draw 6mA?

[sluckey]

I usually guesstimate 1mA or 1.5mA per triode in most cases. But, if you have a schematic and it happens to list the cathode voltage, you can easily calculate the exact current draw. For example, the attached schematic clip of an AB763 Twin Reverb shows 2.0V across a 1500Ω cathode resistor. That calculates to 2/1500=1.3mA. A PI or cathode follower may be more, but if the cathode voltage is known, you calculate current just like the simple preamp triode.

[Rontone]

Ah nice, thanks for that, I am actually looking at the Twin Reverb AB763 to model this preamp on as it has the middle pot and bright switch,

My power supply schematic at the top, I have 2x 12AX7's on node D, so for example that is 1.3mA p/triode = 5.2mA

Resistor between C and D is 6800 ohm x 0.0052 Amps Current Draw = 35.36V drop between node C and D, is that right?

[Rontone]

While I wait to get all the parts together, I prepared the board a bit, a few more component anchoring points were needed, I was going to use eyelets and then thought if I drop the spacers and lower the PCB, vintage purists and collectors look away 

I can fit turrets and do all wiring topside, much easier for me to modify and service in the future, the PCB traces for the preamp power and the preamp channel signal outputs connect well with the turrets, a component leg will go down through the turret and attach to the PCB track to ensure a good connection

Also the PCB was very high anyway, the cabinet handle bolts were protruding, the first time I opened it I didn't realise and when the chassis is slid in and out of the head cab, those bolts were really badly catching on a few caps, really hitting them hard so not good, so cabinet bolts are now shortened too...

Ran out of turrets near the end, but can start assembling some parts to size up and fit, all original components came out pretty easy and were saved, most caps test well for value, but unsure about DC leakage

The PCB is well made, not too brittle material, good large holes on the pads and could fit 4 component legs through, but some pads were missing, seems to be factory original problems, no repair work I can see, the bias circuit area was just hanging there underneath twisted and soldered

I found a cap with 'Nashville Electronics' on it when scrapping some Hammonds, will it give me authentic toan?

[Rontone]

Been sat at home a lot with a bad shoulder and spent some evenings slowly preparing this board,

I need some help working out the power section and what changes to do, should I implement a standby switch with it having this type of bias supply, or leave it as is?

The bias supply resistors are just placed to look at the layout and work out wiring, I believe I will need the true HT figures when my TX arrives to work out the bias dropper resistor?