Mat Fitzwilliam
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Mat Fitzwilliam's Discussions

1650T Brentwood - It lives Igor!

Started Jan 27 0 Replies

Hi Terry, Finally I had time to finish off the Brentwood and get back to you.Thanks for the Aztec circuit diagram, they are almost the same circuits. The Brentwood uses a 6AX5 rectifier whilst the…Continue

Brentwood update

Started this discussion. Last reply by Mat Fitzwilliam Oct 8, 2011. 16 Replies

Hi Terry,Tidying up the power section of the schematic. Two things noted so far.The C/T for the heaters are connected to Pin 8 of the first 6L6 and then it goes to ground via the bias resistor.…Continue

Supro 1650T Brentwood

Started this discussion. Last reply by Terry Dobbs Aug 17, 2011. 3 Replies

Just picked up a well used Brentwood here in Australia. Very rare anywhere but especially here - I'd love to know its' road story! Went through and replaced all of the wax capacitors - some had split…Continue

 

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Mat Fitzwilliam posted a discussion

1650T Brentwood - It lives Igor!

Hi Terry, Finally I had time to finish off the Brentwood and get back to you.Thanks for the Aztec circuit diagram, they are almost the same circuits. The Brentwood uses a 6AX5 rectifier whilst the Aztec appears to have a 5 series. I have amended the circuit diagram to reflect the diffrent rectifier and speaker configuration and added the resistor values for the inputs.In the end I basically rebuilt the whole amp and learnt heaps in doing so. The speakers were shot so I found a pair of 6 x 9"s…See More
Jan 27
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Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Hi Terry,   Sorry to be a nuisance, still no luck with the pre-amps.   5879 - The plate and grid resistors all measure within spec (and I have replaced them with measured spares just to be sure) but the voltage drop over the 470K…"
Oct 8, 2011
Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Hi Terry,   Well progress is made.   Voltage drop in the heater circuit so to check the transformer with no load I disconnected the circuit at the taps and the voltage there was good and held for 15 minutes with little or no…"
Oct 2, 2011
Terry Dobbs replied to Mat Fitzwilliam's discussion Brentwood update
"The heater draw for the tubes are: 6AX5   1.2 amps 6L6       .9 6L6       .9 6sc7     .3 6SQ7    .3 6SJ7     .3 5879   .15 Your…"
Sep 18, 2011
Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Hi Terry,   Just ran voltages pin to pin as you suggested.   On start up 6AX5 pin 2 - 7 Heater   6.5VAC            pin 3 -5 HV Secondary 698 VAC (looks good) 15 minutes…"
Sep 17, 2011
Terry Dobbs replied to Mat Fitzwilliam's discussion Brentwood update
"Absolutely, if the primary or secondary has some shorted windings, it will cause lower voltages. When you read the voltages on the heaters, be sure to measure from, for instance, pin 7 to pin 2 of the 6L6, not from pin 2 or pin 7 and ground then see…"
Sep 17, 2011
Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Hi Terry,   The Aztec is essentially the same circuit except it shows  a 5XX rectifier based upon the pin out.   Finally got some quality time to spend with the Brentwood - interesting. Noting the voltage issues above I checked…"
Sep 17, 2011
Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Thanks Terry,   That looks very similar. I roughed out the pre-amp section last night so I'll compare them this evening. Certainly on first glance the power section looks the same.   Cheers   Mat"
Sep 12, 2011
Terry Dobbs replied to Mat Fitzwilliam's discussion Brentwood update
"Does your amp look like this schematic?"
Sep 12, 2011
Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Ran some voltages tonight to get a rough idea of where it is at. All the caps have been replaced at the start of the refurb as they were shot. So my issues (I guess) will be resistor based.   Anyway here's what I…"
Sep 12, 2011
Terry Dobbs replied to Mat Fitzwilliam's discussion Brentwood update
"On the 5879 plate (pin 8) you might expect 220v - 240v or so and the screen grid (pin 7) you might see around 190v - 210v. On the 6sj7 you might get 60v-70v on the plate (pin 8) and maybe around 30v on the screen grid (pin 6) all depending on the…"
Sep 8, 2011
Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Latest update   Lowered the resistor between node 1 and 2 down to 2K3 (actual) ohms (after some trials). Much livelier, still need to work on the pre-ampo section as it's now louder and more dynamic but still distorted. Anyway for…"
Sep 8, 2011
Terry Dobbs replied to Mat Fitzwilliam's discussion Brentwood update
"The plate supply voltage is about 346vdc according to your chart, the screen grid voltage at 281vdc on node #2 is about 65vdc lower than the plate supply voltage. That is quite a bit lower than the plate supply voltage and might be too low for best…"
Sep 6, 2011
Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Hi Terry,   Sorry my typo, there is a 270K resistor going to ground from Pin 5.   With the voltage at node #2 what sort of value should I be looking for?   Thanks again for all your help. I really enjoy finding out how these amps work…"
Sep 6, 2011
Terry Dobbs replied to Mat Fitzwilliam's discussion Brentwood update
"Keep us posted and feel free to ask any questions, Terry"
Sep 4, 2011
Mat Fitzwilliam replied to Mat Fitzwilliam's discussion Brentwood update
"Thanks Terry,   I know what  I will be up to after work this week...   Much appreciated"
Sep 4, 2011

Profile Information

Do you own a Valco amp? If so, what model(s)?
Supro 1650T Brentwood
What is your favorite guitar?
Fender Nocaster

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History of Valco

The Valco company has its roots in the National String Instrument Corporation, which was founded in 1927. The company is famous as the first manufacturer of resonator guitars, which were hugely popular in blues and (a bit later) bluegrass music. National merged with the Dobro company, another maker of resonator guitars, around 1932 to form the National Dobro Corporation. The company began producing electric instruments in the 1930s that included electric guitars, lap steels, mandolins and amplifiers. These pre-war electric instruments are fairly rare today, though the lap steels pop up with some regularity. The archtop bodies for the guitars were sourced from Regal and then from Kay, but the electronics were developed and manufactured by National-Dobro. The Supro brand name was introduced in the mid-‘30s for cheaper electric instruments. Read More >

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