don't understand this bizzare rectifyer circuit in this Tesla Mono 130 tube amp - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 12, 2020, 12:28:09 pm
content: so I bought this Tesla Mono 130 tube amp for pennies, I'm gonna use the Power transformer and OPT for my guitar amp build.but I don't understand this rectifier circuit, it seems it has a Voltage Doubler circuit for output plate B+ and a half-wave rectifier for the preamp supply from the same winding?I need -200V Bias supply for my circuit design, that I need to draw about 50mA max, but I don't see how I could get that from this transformer.here's the power supply part of the amp:[ Attachment Invalid Or Does Not Exist ]I've also attached the whole schematic of the amp.the whole amp is a bit bizarre, like a fuse for the heater windings, bias supply, but no fuse for the B+.PS. the "G2" capacitors are 200uF. G1 - 100uF, etc.PPS. pin 15 of the transformer (core) is connected to ground, that wire in bold is ground!

author: Ian.M
author_rank: Super Contributor
timestamp: July 12, 2020, 03:23:19 pm
content: Its obvious what its doing if you ignore terminal 7, U3 etc. - its a simple voltage doubler using C301, C302, U1, U2 and winding 6-8. The extra tap '7' on the winding derives a lower output voltage via U3 because it doesn't swing as high as '8'.However the voltages marked on the schematic are as clear as mud,specifically the AC voltages between transformer pins 6 & 7, and 7 & 8. If the latter is actually 545V RMS, the HT rail from U1/C301 would actually be up around 1.5KV !!!The rubric:Code:POZNÁMKY:STEJNOSMĚRNÁ NAPĚTÍ MĚŘENA PROTI KOSTŘE NEVYBUZENÉHO ZESILOVAČE (BOD"7") PŘÍSROJEM AVOMET II. S OZNAČENÍM "+" NEBO ÚDAJE STŘÍDAVÝCH NAPĚTÍ S OZNAČENÍM "~" BYLY ZÍSKÁNY MĚŘENÍM NA VYBUZENÉM ZESILOVAČI NA JMENOVITÝ VÝSTUPNÍ VÝKON.is some help as that (google) translates to:Quotewhich clears up some of the confusion.Assuming the no signal voltages (no suffix) reflect the peak AC voltages of the windings feeding their rectifiers, that gives 6 & 7, 125V RMS (177V Peak), and 6 & 8 177V RMS (250V Peak).Probably the best bet for your negative bias supply would be a capacitively coupled charge pump driven by pin 8 (giving you -250V at no load), then regulate it down to approx -200V, with a string of six 33V Zeners setting the base voltage of a PNP pass transistor. Size the charge pump coupling capacitor so the unregulated voltage drops to about -210V @50mA load to keep the pass transistor dissipation reasonable. Ideally you should feed the Zeners from a current source, e.g a depletion mode N-MOSFET in the classic JFET current sink circuit, with a source resistor selected on test for 1mA through it.If you want better regulation, replace the Zeners with a TLV431 shunt regulator cascoded with a HV N-MOSFET with its gate biassed from a tap in the '431's feedback divider.

author: Ian.M
author_rank: Super Contributor
timestamp: July 12, 2020, 03:47:42 pm
content: Nope there's no hope of getting 700V HT from those secondaries.To eliminate doubt, measure the actual AC RMS voltages between pin 6, and pins 7 and 8.If you don't need the -46V bias supply, you could connect its nom. 35V RMS secondary in series with pin 6 of the doubler secondary(check the phasing!), to boost both the doubler outputs by approx 100V, but that only gets you up to about 600V with no signal, dropping to around 560V at full load. I'd also be suspicious of its current capability given that the 35V winding is fused at only 0.4A, fusing currents reflect RMS ratings, and in a doubler configuration 0.4A RMS only gives you 0.25A DC out.