The Differences between the GZ33, 34, 37

Bob Latino wrote:Hi,

 Rectifiers seldom make any significant difference in sound quality on the ST-120 UNLESS you play the amp loudly or have inefficient speakers. If you play the amp loud a lot OR your speakers are relatively inefficient (87 dB or lower) then I recommend the Weber WZ68 solid state rectifier which can flow more current than any tube rectifier.

Re: GZ33 or GZ37 rectifier tubes vs. a GZ34. GZ33 or GZ37 rectifier tubes have some disadvantages.

1. They are much more expensive than a GZ34
2. There are few vendors for these tubes
3. These tubes are taller (5 1/4" high) so that if you use a tube cage they won't fit under the stock ST-70/ST-120 tube cage.

On the plus side both a GZ33 and a GZ37 with a choke input, as on the ST-120, can flow more current than a GZ34. I have two Mullard GZ33's that I have been swapping in and out with the Weber WZ68 SS rectifiers in the two prototype M-125 amps. I honestly can't hear any difference in the sound of either monoblock.

There is a vendor on Ebay called "Langrex" who sells a NOS Mullard GZ33 for $78.60 + shipping from the UK. I bought two for the M-125's from him and they came quickly and were packed well. This is about the lowest price I have found for a NOS Mullard GZ** rectifier tube.

Bob

Good evening, Bob. I have some questions.

I found this old post of yours from 2010 (a long time ago lol) because I was looking into what the differences were between the GZ33, GZ34/5AR4, and the GZ37 if these tubes are all interchangeable without any modifications or updates to the circuit of the VTA ST-120 Amp ?

I have found the voltage drop with the GZ33 to be about the same as the GZ34.

The GZ 37 has a drop of about 60 volts compared to the GZ33/34.

ok

Bob Latino wrote:Tom,

I am not sure what the exact difference is between the GZ33 and GZ37. I believe that the GZ37 requires a lower input cap value on the quad cap than the GZ33 but customers tell me that both work well in the VTA amps. These two tubes are very rugged and can deal with quick ON/OFF/ON "short cycling" better than modern GZ34's. Upscale Audio in CA still sells them for $100 a copy. Yes - they are expensive buy they will probably outlast three new production 5AR4's. Upscale Audio link below ..

GZ33 for sale at Upscale Audio

Bob

Thanks Bob....

That's what I was hoping to hear because to me a more reliable tube rectifier means less chances of problems of short circuiting and from what you say it's more or less 3 to 1 as far as the service life of those tubes and to me that's well worth the extra cost. Also I have a bit of tube envy on that Coke bottle shape of those tubes.

Like I said to you before I bought the 120 I was going to buy the 125 but the wattage was just too much for where I'm living and it's almost the same case for the 120. I don't need the copper cap because I'm just not going to blast that 60 W to death. Klipsch makes some really good efficient speakers and that's what I intend to use.

I also believe I have figured out a workaround to add 1/2inch or more to the bottom of the old Dynaco St 70 cage so it will work.

Pablo wrote:I have found the voltage drop with the GZ33 to be about the same as the GZ34.

The GZ 37 has a drop of about 60 volts compared to the GZ33/34.

Exactly. I discovered the GZ33 to be a drop in replacement for the 5AR4. In fact I replaced a 5AR4 with a GZ33 and didn't even need to readjust the bias!

The GZ37 has about the same drop as a 5U4 but with a slow warmup.

I settled on a 5AU4 for the rectifier. Handles 100MA more current than a GZ34, is dirt cheap for OEM tubes, and survived a shorted output tube that had the rectifier seriously red plated. Draws more filament current but Bob said the 5v winding on the ST120 is rated at 4A.

bobgroger wrote:I settled on a 5AU4 for the rectifier. Handles 100MA more current than a GZ34, is dirt cheap for OEM tubes, and survived a shorted output tube that had the rectifier seriously red plated. Draws more filament current but Bob said the 5v winding on the ST120 is rated at 4A.

Good choice. It will most likely last a very long time.

bobgroger wrote:I settled on a 5AU4 for the rectifier. Handles 100MA more current than a GZ34, is dirt cheap for OEM tubes, and survived a shorted output tube that had the rectifier seriously red plated. Draws more filament current but Bob said the 5v winding on the ST120 is rated at 4A.

Yeah, those nos 5au4/5v3/5aw4/5u4gb tubes are very good and can be had quite cheaply because most people think of them as tubes for old television use rather than hifi. They're super long lasting and durable because they were meant to last thousands of hours in televisions under heavy use.