What tube amp to build.

[OldHector]

I heartily recommend the Tubelab offerings. He has his own vendor forum here, and the support from George and the community is second to none.

You have three entry points, depending on your ambition.

I use the Simple Single Ended (SSE) all the time, and have never had power issues driving my medium sensitivity bookshelf speakers. With transformers from Toroidy, frequency range is excellent, and if you take the transformers without the fancy covers, all transformers can be had for £200, which will be your single biggest outlay for a tube amp.

[Bas Horneman]

I'm with Eli here. What speakers will be used? But also, what size is the room they will be used in?

[Jeff429]

Room is small say 10x12. Have not picked speakers yet.
Was thinking about PSB X1T or X2T or KEF Q750.
Will a class A drive these? How low db speakers would I need?
I don't want to get into the engineering side of this. I would like to find some proven prints to build from. I am open to any speaker suggestions.
I have never listened to a 5 watt amp before, is that enough power to to have some volume out?
A kit would be OK or prints with a BOM. The small parts are no problem. I would need advice on transformers.
Would you use a design that used tubes in the power supply or a SS power supply?

[evanc]

I’m going to agree with the earlier advice to build a simple single ended amp. Get a copy of Morgan jones book valve amplifiers and gain a true understanding of what’s happening. It will give you building and theoretical expertise while still leaving room for some experimentation. I wish I had started this way.

[Eli Duttman]

Originally Posted by Jeff429

Room is small say 10x12. Have not picked speakers yet.
Was thinking about PSB X1T or X2T or KEF Q750.
Will a class A drive these? How low db speakers would I need?
I don't want to get into the engineering side of this. I would like to find some proven prints to build from. I am open to any speaker suggestions.
I have never listened to a 5 watt amp before, is that enough power to to have some volume out?
A kit would be OK or prints with a BOM. The small parts are no problem. I would need advice on transformers.
Would you use a design that used tubes in the power supply or a SS power supply?

If you want some volume out, apply Paul Joppa's 102 dB. rule, which states that an amp/speaker combo should be capable of a 102 dB. SPL at a 1 M. distance. Speakers whose sensitivity is upwards of 96 dB. are in order with a 5 WPC amp. Also, the speakers must exhibit a reasonably flat impedance curve. You can use nominal 8 Ω speakers, with impedance dips, on 4 Ω O/P transformer taps. However, you have to derate the claimed sensitivity by 3 dB.

Let's examine the KEF Q750. Its nominal impedance is 8 Ω and the claimed sensitivity is 88 dB., but it dips down to 3.2 Ω. Therefore, when mating KEF Q750s to a tube amp its impedance is 4 Ω and the sensitivity is 85 dB. Joppa's Rule indicates that (sic) upwards of 50 WPC are in order. That's push/pull KT88 pair "territory".

Most, but not all, commercial speaker designs are not suitable for mating to low power tube amps, either SE or PP. Klipsch "Heresy" and "Forte" speakers will be fine with 5 WPC, provided a decent damping factor is present.

[Tom Kamphuys]

Are there any other speakers, more 'El Cheapo', you could advise?

[cogitech]

In a 10 x 12 room a 5-7w amp will create plenty of volume with speakers as low as 88-89dB efficiency.

Hell, even in my much larger room, my Tubelab SSE has no problem hitting over 95dB peaks with my two 88-89dB speakers at a distance of 2 meters. Way too loud to have a conversation, and loud enough to cause cumulative hearing loss.

[Eli Duttman]

Originally Posted by Tom Kamphuys

Are there any other speakers, more 'El Cheapo', you could advise?

High sensitivity and low cost seem to be incompatible. The "sweet spot" may be mid 90s sensitivity and 8 or so WPC. Low 90s sensitivity and a "knock off" of the Sherwood S5000's power section (20 WPC) could be attractive too. 7189 equivalents are needed by the S5000 and the nice Russian 6Π14Π-EB (6p14p-ev), AKA EL84M, affordably meets the requirement.

It's a stone cold fact that in any power handling range SE O/P "iron" costs more than the PP stuff. Like it or not, "iron" is a major expense in building tube amps.

[Steve Morley]

Jeff,
A straight forward and fairly simple amp is the Tubelab SPP (Tubelab forum on this site and circuit board available from Tubelab.com). ~15wpc from one 5AR4, four EL84s and two 12AT7s. If you are in the USA a complete set of "iron" is available from Edcor for about 200 bucks.

Steve

[Jeff429]

Thanks for all the info. I will research the tube labs.