Voltage Chart for the Tweed 5f2-a

[marcbo77]

Hi there,

A neighbour and friend decided to build from scratch a Fender Princeton Tweed based on a DIY chapter in a guitar amp book. The design looks to be reasonably faithful to the original 5f2-a schematic. When he plugged in the tubes he said that the output tube "popped". He asked for my help and so far I have taken a cursory look. ( I used to rebuild tube-based ham radios in my teens; alas that was 30 years ago!). When I plugged in the rectifier tube - it seemed to me that all of the voltages were a bit "hot". Perhaps that is OK - but I would not mind doing a sanity check with some comparative data. I've found plenty of of schematics for the 5f2-a on the Internet - but none that have reference voltages included with them. Would any of you have that kicking around ?

Thanks,

Marc,
Toronto

[MWJB]

Voltage at 6V6 pin 3 might be anywhere from 330vdc up to 410vdc, or a shade over, without knowing power transformer spec it's hard to know. Pin 4 might be 20-30v less than that.

It would be good to know the voltage at 6V6 pin 8 too.

If the black/grey metal plate in the 6V6 is glowing red/orange then it's running too hot.

Voltage at 12AX7 pin 1 & 6 might be as high as 220-230vdc and still considered within spec?

Does the amp make sound with anm instrument plugged in?

How loud was the pop? Just having the tube pin 3 contact the OT primary might cause a little pop/click in the speaker.

[marcbo77]

Thanks for the test tips MWJB. The Power transfomer without load had an ouput of about AC 720Volts which seemed high. (Can't remember the brand - but it did not come with a spec sheet). My neighbour was at band practice last night - so I'll take a poke around tonight.

Thanks,

Marc

Voltage at 6V6 pin 3 might be anywhere from 330vdc up to 410vdc, or a shade over, without knowing power transformer spec it's hard to know. Pin 4 might be 20-30v less than that.

It would be good to know the voltage at 6V6 pin 8 too.

If the black/grey metal plate in the 6V6 is glowing red/orange then it's running too hot.

Voltage at 12AX7 pin 1 & 6 might be as high as 220-230vdc and still considered within spec?

Does the amp make sound with anm instrument plugged in?

How loud was the pop? Just having the tube pin 3 contact the OT primary might cause a little pop/click in the speaker.

[MWJB]

"The Power transfomer without load had an ouput of about AC 720Volts which seemed high." You mentioned a "fitting" a rectifier, so I'm assuming it was a tube - in that case you might get 720VAC end to end at the secondary but there should be a grounded centre tap, making 360-0-360VAC...which seems about ball park to me. Voltage will drop under load, which is how readings should be taken.

[marcbo77]

MWJB - Thanks for the feedback. So I spent a few hours playing around with this DIY project without much success. To your point the circuit voltages dropped noticeably when all three tubes were plugged in. I now doubt that the issue is with the transformer being over-voltage - but I am not discounting it altogether.

The circuit is based on the DIY project in Chapter 9 of the The Guitar Amp Handbook by Dave Hunter. I have to commend my neighbour - he very carefully followed the instructions and all of the connections and capacitor polarities etc. look to be correct. HOWEVER; when you power up the amp, the DC voltages feeding the output tube (6L6) on the grid and cathode pins drop quickly. Then the 6L6 oscillates audibly and rather loudly. (Note: I have not even plugged in an output speaker yet!) I cannot take a voltage reading on the plate pin (3) at all - either on a DC or AC setting on my voltmeter. I suspect that the tube is injecting so much noise ( I can generate a small arc between the pin and the meter probe) that the meter is not able to make a measurement.

I experienced the same result when I substituted the 6L6 with an EL34. Hence my suspicion that this is something more than a PT producing a high plate voltage.

Any comments appreciated.

Marc,
Toronto

"The Power transfomer without load had an ouput of about AC 720Volts which seemed high." You mentioned a "fitting" a rectifier, so I'm assuming it was a tube - in that case you might get 720VAC end to end at the secondary but there should be a grounded centre tap, making 360-0-360VAC...which seems about ball park to me. Voltage will drop under load, which is how readings should be taken.

[MWJB]

Try switching the primaries (B+ & pin 3 connections) for the output transformer. The polarity as you have it may be making your "negative feedback" positive?

Can you post any pics of the amp in question (not schematics, or links to layouts)?

[marcbo77]

Sure - I'll post some pics tonight. Presumably there is some facility to do so on this site. I'll also check the polarities as you suggest.

Marc

Try switching the primaries (B+ & pin 3 connections) for the output transformer. The polarity as you have it may be making your "negative feedback" positive?

Can you post any pics of the amp in question (not schematics, or links to layouts)?

[marcbo77]

Hey there MWJB - I have not given up! Unfortunately other priorities have taken me away from this. I'll get to debugging this on Thursday.

Marc

Try switching the primaries (B+ & pin 3 connections) for the output transformer. The polarity as you have it may be making your "negative feedback" positive?

Can you post any pics of the amp in question (not schematics, or links to layouts)?

[stingray_65]

http://www.schematicheaven.com/fende...6g10_schem.pdf

altho its labled as a "Harvard 6G10" the schematic is identical to the Princeton 5F2A

I used these voltages on my first Princeton build with great success.

Mind you the voltages were called out with the amp supplied with 110V mains and current main voltages here are now 120V so all my readings were about 10% higher than called out, but that was to be expected and falls well within the stated + or - 20% tolerance.

If you wish to lower voltages to the vintage correct levels, the reversed zener diode works well, but I found it made little difference.

OH! I did have to lower my heater supply voltage so that i wasnt cooking them. this was as simple as adding a diode on one leg of the heater supply. the added benefit was DC heaters wich left no hum at all.

Ray

[davohilts]

high voltages across the board

Hey guys!
I'm building this same circuit and had the same oscillation issue which I solved by switching the secondaries on the OT side. The problem I'm having is hum which i'm sure is caused by the sloppy dressing job I did on the wiring but the real issue is my voltages.
On the 6V6 they are as follows:
p3=417
p4=373
p5=175mv
p6=24
all heater pins measured 3.3 vac

I also took voltage reading along the power rail starting with the first filter cap:
16uf=417
16uf=416
8uf= 363
8uf=291

I did some modification using 2 diodes in stead of the rectifier tube.
I also ran twisted pairs of heater wires to the tubes and ran 100 ohm resisters from each pin on the pilot lamp to ground.

My amp is sounding better after switching the secondaries of the OT but the tone pot when turned down all the way starts the amp squealing. I can turn it to treble slightly and it goes away. Is it a bad pot?
I've got pics I can post it that would help explain myself if necessary.

[stingray_65]

I have built sevral of these for friends,

this circuit I have found to be extremley fussy as far as layout and dress are concerned.

I have found it best to run the wires (with the eception of the heaters) exactly the same way that is in the layout diagram.

http://www.schematicheaven.com/fende...5f2a_schem.pdf

the exeption being the heaters as I mentioned before, from your pilot lamp where your 100R resistors are run a twisted pair along the side of the tagboard to the rear corner of the chassis. then follow the chassis along the corner branching up to the sockets where apropriate.

I had one version I built give me nothing but fits after I thought I had found a more efficient way to lay it out.

Just my .02 take it for what it's worth

Ray

[davohilts]

thanks Ray! Sounds like good advice.
dave

[MWJB]

175mA on pin 5 seems high, should be "0v". Check integrity of ground connection on the 220K resistor connected to pin 5. Other than that voltages look OK, if you didn't want slightly elevated voltages, then you wouldn't have used a SS rectifier.

Shouldn't be a voltage drop between the 2x16uf caps as these are connected to each other.

Yes, pics would be very useful. If you are aware that your wiring is messy, tidy it up. Keep grid wires (12AX7 pins 2 & 7, 6V6 pin 5) short & away from heater & plate wires (12AX7 pins 1 & 6). Ideally wire from input jack to 12AX7 pin 2 and wire from vol/tone controls to 12AX7 pin 7 should be shielded, ground the shielding at one end only, heatshrink the other to avoid shorts.