My custom VTA-120

I had a custom VTA 120 built. I eliminated the Triode/Pentode switch, the ugly quad can cap, and redesigned the piwer supply. The single GZ34 just is inadequate. It operates at almost 100% of its capacity , and is prone to failure.

I replaced it with 2 parallel wired 274B , mesh plate rectifiers from Emission Labs. You get the following benefits:

1. Current is 320-340mA
2. This tube can get 500-550 volts to power tubes. It is built like s tank.
3. Paralell circuit reduces impedance by 50%
4. Tube lag is virtually non existent. Responsiveness more like solid state
5. B + increased to about 500 V
6. I run a quad of Mullard Kt88s
7. Amp is super quiet, no hum, noise, static.
8. Sound is rich, warm and deep with lot of detail
9. I use Sylvania 5963 , 3 mica gold brands in V1 V3 phase inverters
10. Input tube is Genelex 12AU7 gold pin.
11. Mesh plate EML are slow to warm, protect power tubes from high voltage beforr they warm up.
All caps are mounted under the chassis.
No missing the quad cap.
.B+ may allow kt90 or kt-120

Pictures, please.

(Caveat: I'm NOT a tech guy.)

This seems like a really good idea. I've got a VTA ST 70 that I've had for about 8 years and no problem with the GZ34 in that but over the years it seems that alot of posts about the ST120 involve rectifier connected problems.
Since you ditched the cap can you have the space for another rectifier. I'd like to see a pic also. I bet it looks cool.

I used to have a Vox AC 100 guitar amp that ran four EL 34 and was rectified with a GZ34. It seemed to work fine and loads of '60's rock bands used them but I did have some comments on the web that the GZ34 was "on the edge".

Slightly off topic: From internet reading (risky) it seems to me that if an amp calls for a 274B it's okay to use a 5U4G but you would have to be careful and check some capacitance if going from a 5U4G to a 274B. Does this sound right?

(My current main amps are 300B mono blocks rectified with 5U4G but they originally came with 274B. They sound really nice.)

Doing dual 274B requirerd s few mods, nothing hard
1. The first cap connected to the tube must be 8uf or less. You can add higher subsequently but keep cap 2 at 20uf or

You can use dual 5V4G in the St70 and 274b in VTA120. Note, you must keep the total heater current at 5 amps or less . You can’t use dual 5U4 or 5z3, they draw 3 amps in heater.

You Alzo need to add resistors when parallel rectifiers But the mod is easy

If you want to run dual rectifiers, you must do the following:
1. Pick a good vintage NOS, brand( Sylvania, Mullard, Telefunken, )
2. Indirect heating work best( note: PS Vane 274B are NOT recommended; they c
Direct heated and wallop the lower tubes with full B+ while still cold.)
3. Check current capacity AND voltage of new rectifier. TheGZ34 can supply 260
mA . Its a goid tube, but if you be rely on obe innSt7”

4. Compare the single GZ34( or 5R4) to amps Current use with 4 el34s. And 3 —12AU7s you need about 50-55mA eacH, and the 3
preamp tubed use about 10mA, so depending on class and push pull, an ST70 needs About 230- 250 mA dc current. The GZ34 will be at 250/260=96.15 % of its maximum!!
and this current is about 430 volts; if you try to hit thr power tubes at 500v DC, the GZ34 can only produces about 220mA, ehen thst happens, say hello to red plates and a burnt tube.

Now, if you use 2. 5V4G, each needs just 2 amps heater current(do parallel they just need 4 amps, the power transformer can supply 5 amps, so far so good, each 5V4G can flow 175mA( but parallel wiring DOUBLES the current, so you have 350 mA available . If we run same conditions as GZ34, your now at just 250/350=71% of capacity. Nice safety margin ? But it gets better . Lets say you want 500 volts dc on B the tube plates . The GZ34 can stretch and make it, but your current will max out at 225 mA. All tubes drop voltage as they are a type of impedance. The GZ 34 drops 17volts throufh the tube, so if you got the 500 volts in, you’ll head into the filter with 485. And it will from there. Maybe you get to the anodes with 465vDC. Not bad. But that overworked GZ34 is a minor voltage spike from Red plate city , you dont want to go there

Meanwhile, two parallel 5V4G each have s
A voltage drop of 25v, but before you say thats higher than the GZ34, remember when you wire resistors in parallel, the combined iimpedance for 2 circuit elements is
equal to 50% less! So two 5V4G have a combined voltage drop if just 12v. That’s 17-12:
/ 17 =29% les voltage drop thE poor GZ34. At just 12 volts loss across two rectifier tubes you enter filter section with about 490+ volts B+. And since your available is 350-/260= 90 mA cushion,
. Tube lag, the seemingy slow response seen in some tube rectified amps is caused.the volltage drop across the rectifier tube. I have found the amp is now much more responsive ( you cut the voltage drop by 30%. So tube lag will be almost nonexistent, yet you dont lose any tube sound! Almost as responsive as Solid state, but none of fhr clinical tone.

To make this mod, let me
Know and I’ll post 4 crucial steps you need (and many s

Amp makers not aware of!)

Good luck