I currently have an Art Audio Quintet power amp. During the past few months my hands have begun itching to build a power amp myself. I've been jumping between designs and tube choices all that time. Because I am now working on Lowther speakers which have a high sensitivity (around 104dB) I do not need a lot of output power. For this reason my choice fell on the famous 300B (later also the 45), and it would be driven by an interstage transformer. I wanted to keep this amp purely DHT, but found some problems along that path. Among these problems was the fact that I couldn't decide between the 45 and 300B as an output tube, and that I couldn't find a driver that could drive these tubes to maximum output withut adding another gain stage in front of it. Below is a compilation of arguments for choosing the tube types and other components.
WARNING : The schematics and values that are mentioned in the texts below are still in an experimental/deisgn phase. I'm not putting any real time into this project yet because I am still working on my preamp and speakers. Please do not build this amp before you check the voltages and component values yourself. I haven't looked for any errors yet but I bet there are a few minor ones in there !
- DRIVER STAGE -
I needed about 120V of grid swing peak-peak on the 45 to drive them to full output. To get this with an input of 2 volts from any line level preamp, it would need a gain of about 25 to 30. The intended 31 which I had in mind as a driver didn't qualify for that job but I wanted only two tubes in my power amp. I started browsing through data books in search of a DHT with a gain of >20 and which could still qualify as a driver. My search turned up nothing that would do the job. I remembered reading an article about a indirectly heated tube named the 5842 which had a gain of 43 and very delicious specs. An ideal tube for my needs, but it wasn't a DHT.
The only solution was to redesign my preamp to give it a higher output. I didn't need to make my power amp "universal" so that any commercial preamp could drive it. This way the input sensitivity didn't need to be restrained to 2 volts. Letting my preamp output something around 15V would take care of the problem. With 15Vrms input the 31 with it's gain of about 4, due to the interstage transformer, could now drive the 45 to it's maximum output.
- THE 45/300B DILEMMA -
A few weeks ago I started thinking about using the 45 or 50 tubes instead of the 300B which I was originaly going to use. The 45 could deliver 2W and had lower B+ and current than the 300B, the 50 could deliver about 5W at roughly the same parameters as the 300B. After seeing the current prices for a NOS 50, which is about the same as that of a NOS 300B (non WE that is), I thought it best not to think any further about this tube. Availability was pretty much nill, and prices reflected that all too eagerly. The 45 on the other hand was still affordable and very much available. The 2 watts that it could muster should be enough for my soon-to-be Lowther horn speakers. But would it really be enough? Since I don't want to find that out after I built the entire amp, this raised some questions as well.
Since I am going to use the Tango XE-60-5SNF for the output, the output impedances for the 300B and 45 are not a problem since this tranformer has a 5K primary which suits both the 45 and 300B. The interstage will also be the same in both cases, namely the Tango NC-14. The only difference between the two would be the bias, the B+ and heater supplies. Everything else would be interchangable. So there is where I came with the idea that if the 45 would not deliver enough power, I would be ble to plug in a 300B instead. This way I can find out which of the two really suits my needs in power and taste and go from there. I'll only need a variable negative bias and a changable B+, the two heater windings is a matter of pennies since I'll be using a AC supply for this. The bias shouldn't be a problem, -76 or -56V, just turn the pot a little bit further. The B+ will take some more work though.
- TUBES -
Last year I purchased several DHT's from International Components and Antique Electronics Supply for a few bucks. I thought they might come in handy some day, and otherwise they would look great laying there on my bookshelf. Among them were 1G4's 1H4's 1J6's 1LE3's 3B7's 30's 31's and several other DHT tubes. This little supply is perfect when you're doing some experiments and deciding which tubes you would like to use in your next project.
- RCA 31 -
The two 31's are from RCA, have an ST envelope and a UX-4 base. I bought these from International Components for about $5.00 each, NOS but relabeld. It has the following parameters:
Ua of 180V
Ia of 12mA
Rp of 3600
S of 1050
mu of 3.8
Because of the interstage transformer, gain for this stage will be slightly higher than the mu of the tube, I figure it will be around 4 times.
- Sovtek 300B -
The 300B's I bought a few months back from New Sensor Corp. for $69 each. I thought it was a bargain at the time and I was already planning to build a 300B amp. Haste makes waste they say, we'll have to see if it's true. I don't think I'll have to place the specs for this tube here. They've been published over and over in every important tube magazine for years. If I need to replace the 45 with this tube 'll be running them at 400V @ 50mA into 5K giving an output of just over 6W.
- #45 -
The Rec.Audio.Tubes newsgroup put me in contact with a few people who were selling used 45's for descent prices. I just ordered 2 pairs of ST bulbs (3 GE's and one Westinghouse tube) from Robert S. Ross in Canada who was nice enough to sell me these used-but-good sets for a nice price. I also bought a set of Radiotron 245 balloons from Coastal Concepts just for comparison. Running parameters for the 45 are as follows...
Ua of 275V
Ia of 36mA
Rp of 1700ohms
S of 2050uA/V
mu of 3.5
Pout of 2W
Rl of 4600ohms, but I'm using a 5K trannie.
- HEATER SUPPLY -
Heater supplies are always a topic by themselves. AC current is prefered because of life and linearity. DC is prefered to eliminate hum on the output. Why not have AC without hum?? A while ago I saw that imaginary lightbulb appear above my head :) I'm still working on it right now, but I don't have a lot of time left over during the week. When I finish building and testing what I have in mind I'll post schematics on this page. It might be good for a surprise... For the moment I'll place schematics for a AC heated driver and power triode. I would prefer linearity over hum, but my next speakers (Lowthers) will have an extremely high efficiency, so this hum will be much more audible than with my current speakers.
That's it for my little story. Below are the schematics and part lists for this amp.
Schematics for the 45 amplifier
Schematic of B+ supply for the 45
Schematic of B+ supply for the 31
Schematic for the negative grid bias for the 45