Initial work and ideas were done in early 2005, creating the basis for two amplifiers - RH88 and RH300B. The RH88 was immediately transformed into breadboard, and it remained in function for the better part of the year. Due to objective reasons, work on the 2nd generation of RH amplifiers was halted, and it was not until the summer of 2012 that the breadboard emerged from a box.
Initial publishing of the schematics for the RH88 has shown that DIY-ers are interested, but fear the high prices of tubes, OPTs, chokes, and other parts associated with this amplifier. In particular, NOS tube prices have soared during the last decade, making both KT88 and 6550 NOS tubes almost collector's items with high price tags.
It is difficult to judge the prices involved when building a 5W class amplifier against the prices of building a 10W+ class amplifier. One should keep in mind that while the RH84 could be built with great results using parts from old console amplifiers, and improved using new parts - it's competitors are maybe some 2A3 or 6B4G amplifiers. The RH88 was conceived to compete with 211/845 amps (in class A1 - which most of those are, although many commercial products are marketed claiming class A2 output power) and easily exceed the power offered by the 300B SE amps... as such, it is still a relatively cheap amplifier to build, depending on the parts used.
The RH88 was initially conceived as a 6550 or KT88 amplifier, and later some DIY-ers asked for a customization to use 8417. While the latter could be inserted in the socket and used without problems, the output power would be lower, and the sound inferior, due to a lower current draw in the same circuit. I was also considering whether EL34 and 6L6 types could be used as well, and eventually decided that the amplifier should become universal - allowing the user to insert almost any given tube which is compatible with the pinout.
The principle is quite simple: regardless of tube type, the cathode current draw remains fixed at 100mA. Since output tubes vary in anode dissipation, with a fixed current draw the voltage across the tube must be variable to allow for different tubes to be used. The variable B+ is achieved by changing rectifier tubes (in this case, to simplify everything and avoid mistakes, rectification is done by hybrid Graetz bridge). Basically, a 5AR4 (GZ34) will give approximately 45V of DC more than a 5R4, which creates a difference of, roughly, 4.5W of dissipation. Some tubes draw less current through the anode, since they draw more current through the second grid (i.e. EL34)... which creates a further difference in anode dissipation (anode current x voltage across tube). Last but not least, tubes will differ in the cathode voltage at the same current draw (EL34 and 8417 approx. 10V while 6550 will have approx. 22V) which creates a further difference in anode dissipation (different voltage across tube).
As a rule of thumb, in the above circuit EL34 can be used only with 5R4 rectifiers, while GZ34/5AR4 should be used only with tubes capable of more than 30W anode dissipation (6550, KT88, 8417). It goes without saying that nothing forbids the user to combine the latter tubes with a 5U4 or even 5R4 - where of course the lower voltage across tube will result in a slightly lower output power. It can be thus said that output power varies between 8W per channel and 12W per channel.
The above picture: 7027A RCA tall bottle in action with 5AS4 GE and 6201 Philips driver.
In order to use EL34, it is necessary to connect pins 1 and 8 - which does not represent a problem to most tubes, except the 7027 which has a second connection to grid No. 1 on pin 1. A switch will allow the user to connect or disconnect the two pins on the output sockets (part of the switch can be seen on the above picture, choosing between the "7027" and the "EL34" setting.
Besides allowing the user to use a wide variety of tubes (basically, what is at hand), the amplifier is particularly suited for playing with tubes (tube rolling). While the basic character of the sound remains the RH signature, additional nuances can be had from different combination of output tubes, rectifiers, and even different drivers (ECC82 can be used with 8417 and EL34, but not with 6550 and KT88).
I prefer to let others speak about the sound of the RH amps...
