Hello, I'm looking to get a tube amp, however I know just about nothing about electronics and have about the same absence in knowledge of valves. (I'm not generally stupid just ignorant in electronics - if that clarifies things!)
So I'm thinking the learning curve in electronics is going to be too great initially for me to go and build an amp from scratch and I'm a very impatient guy, so I was thinking about buying a Leak Stereo 20 (that worked and was serviced etc.) this would get me going and I could then learn more from there!
However again I've had a quick look at the Leak Stereo 20 and I can't even see what the switch cable is for and why some have changed it for a toggle switch and where the inputs are and whats the connector at the front...... I know this makes me probably sound quite simple to you experienced bunch, but is this a good way forward (buying the Leak 20) and if so is there an instruction book somewhere to show me how its suppossed to be set up.
NB Will the Leak be OK for powering my KEF floorstanding speakers or would I need something in between (or new speakers!)
Many thanks for any help..
So I'm thinking the learning curve in electronics is going to be too great initially for me to go and build an amp from scratch and I'm a very impatient guy, so I was thinking about buying a Leak Stereo 20 (that worked and was serviced etc.) this would get me going and I could then learn more from there!
However again I've had a quick look at the Leak Stereo 20 and I can't even see what the switch cable is for and why some have changed it for a toggle switch and where the inputs are and whats the connector at the front...... I know this makes me probably sound quite simple to you experienced bunch, but is this a good way forward (buying the Leak 20) and if so is there an instruction book somewhere to show me how its suppossed to be set up.
NB Will the Leak be OK for powering my KEF floorstanding speakers or would I need something in between (or new speakers!)
Many thanks for any help..
I'd suggest that you listen to a decent ST20 before making a decision. For all the hype, they're not all that wonderful IMHO, and are overpriced these days - you might well get a better deal with a modern Far Eastern model, or even a pair of Mullard 5-10s, which sound much better to me.
To answer your other questions, the 'umbilical' connector and switch connector were intended for use with Leak's preamps or similar, most of which are by modern standards very mediocre, and not worth using. You can fit a separate switch across the switch terminals, or bridge them and switch the supply to the amp however you choose. The inputs should go to the phono sockets on top of the chassis, which are paralleled with the umbilical inputs. You may need to update these sockets, or obtain vintage style 'long pin' phono plugs to suit. Similarly, the amp uses difficult to obtain speaker connectors (at least in some versions), and are better changed for standard 4mm sockets / binding posts.
For normal domestic purposes, the ST20 will be quite adequate for KEFs of a similar vintage.
To answer your other questions, the 'umbilical' connector and switch connector were intended for use with Leak's preamps or similar, most of which are by modern standards very mediocre, and not worth using. You can fit a separate switch across the switch terminals, or bridge them and switch the supply to the amp however you choose. The inputs should go to the phono sockets on top of the chassis, which are paralleled with the umbilical inputs. You may need to update these sockets, or obtain vintage style 'long pin' phono plugs to suit. Similarly, the amp uses difficult to obtain speaker connectors (at least in some versions), and are better changed for standard 4mm sockets / binding posts.
For normal domestic purposes, the ST20 will be quite adequate for KEFs of a similar vintage.
iant187,
Something to be aware of about the Leak amp is that it has WAY too much gain to be used with a CDP as the source. Dealing with that issue is not particularly difficult. Since learning how to read a schematic is something you MUST do, I'm linking the ST20 diagram. Look here. Please observe that a 12AX7/ECC83 is shared between the I/Ps of both channels. Replacement of that tube by a 12AY7 will reduce overall gain. Unfortunately, the reduction is likely to be insufficient. Buy a 50 KOhm stereo volume control and a pair of 1% tolerance 47 KOhm resistors. Connect each of the "hot" wires from the CDP to an end of a 47 KOhm resistor. Connect each remaining resistor end to the "top" terminal of the control in each of the channels. The wipers of the control connect to the "hot" terminals of the amp. In each channel: the "cold" (ground) wire from the CDP, the "bottom" of the control, and the "cold" wire to the amp are connected together. Short, LOW capacitance cabling between the volume control and the amp is essential.
Something to be aware of about the Leak amp is that it has WAY too much gain to be used with a CDP as the source. Dealing with that issue is not particularly difficult. Since learning how to read a schematic is something you MUST do, I'm linking the ST20 diagram. Look here. Please observe that a 12AX7/ECC83 is shared between the I/Ps of both channels. Replacement of that tube by a 12AY7 will reduce overall gain. Unfortunately, the reduction is likely to be insufficient. Buy a 50 KOhm stereo volume control and a pair of 1% tolerance 47 KOhm resistors. Connect each of the "hot" wires from the CDP to an end of a 47 KOhm resistor. Connect each remaining resistor end to the "top" terminal of the control in each of the channels. The wipers of the control connect to the "hot" terminals of the amp. In each channel: the "cold" (ground) wire from the CDP, the "bottom" of the control, and the "cold" wire to the amp are connected together. Short, LOW capacitance cabling between the volume control and the amp is essential.
Eli is absolutely right in saying that the Stereo 20 has far too much gain. However, simply replacing the input ECC83 with a lower gain valve will only lower the amount of global negative feedback available, not the closed loop gain. You also need to change the feedback resistors and their associated compensation. And that's rather harder. Frankly, it's not really worth the trouble. You'd do better to treat the Stereo 20 as a chassis with a set of transformers, and build a more modern circuit with a more sensible gain. In the meantime, just use it as is with an attenuator on the input.
Oooo thanks for the replies guys.
Typical of me just wanting to jump sraight in and play with something, I'm glad I took the time to post!
In that case perhaps I'll have a go at building a very basic amp first (I guess there must be a load of posts about that somewhere, but if you have any suggestions I'd appreciate any help).
I was thinking as an experiment to get me going in the world of valve amps, that I'd like something just small and simple that could power my iPod for now - as I said above if you've got any circuit diagrams or suggestions or kits then I'd appreciate any help.
Regards
Typical of me just wanting to jump sraight in and play with something, I'm glad I took the time to post!
In that case perhaps I'll have a go at building a very basic amp first (I guess there must be a load of posts about that somewhere, but if you have any suggestions I'd appreciate any help).
I was thinking as an experiment to get me going in the world of valve amps, that I'd like something just small and simple that could power my iPod for now - as I said above if you've got any circuit diagrams or suggestions or kits then I'd appreciate any help.
Regards
iant187 said:
Dude,
Before you spend a single penny, please tell us the nominal impedance and efficiency ratings of your KEF speakers. Mating the speakers to the wrong amp will cause you considerable grief.
EC8010 is spot on about not using a 12AY7 in the Leak ST20. I overlooked the fact that the tube is inside a global NFB loop. 🙁 IF the ST20 is suitable for mating to your KEF speakers, you will build a passive attenuator along the lines previously mentioned. However, the parts values will change. Buy a 10 KOhm stereo volume control and 90.9 KOhm 1% tolerance fixed resistors. A passive attenuator is an excellent 1st project for a complete "noob".
When you feel ready to attempt your 1st scratch build of an amp, may I immodestly suggest "El Cheapo". Read the lengthy saga here.
I use the Leak TL12.1 and theya re wonderfull I used to have the LeakTL12+ which are really the mono version on the Stereo 20.
I have had a hard time finding something that sounds better tha my Leak tl12.1, by the way they are muuuch better than the Quad II.
About gain issues: Yes its high, good for passive preamp, or volume control attenuator. I use a DIY 1k shunt type, but the EVS 1k attenuator will work wonderfully, no need for a preamp.
I have had a hard time finding something that sounds better tha my Leak tl12.1, by the way they are muuuch better than the Quad II.
About gain issues: Yes its high, good for passive preamp, or volume control attenuator. I use a DIY 1k shunt type, but the EVS 1k attenuator will work wonderfully, no need for a preamp.
Eli Duttman said:
Thanks Eli, The speakers I was going to use with the amp actually are B & W 601 S2's, which are 8 ohm and 90db sensitivity - not sure what my Kefs are! but I thought the B & W would be more suitable as prob a bit more sensiive.
I'm perfectly happy with El cheapo for now, just want to get my hands wet so to speak!)
iant187 said:
According to B&W, the 601S2 is nominally 8 Ohms and is 88 dB. sensitive (2.83 V.). 88 dB. sensitive is bad enough, as upwards of 30 WPC are needed. A most unpleasant specification is the 4.3 Ohm impedance minimum. For mating with tubed amplification, think of the B&W speakers as being 6 Ohm nominal and 86 dB. efficient. You need the power PP KT88s deliver to drive the 601S2s. The "El Cheapo" topology lacks the gain needed to drive KT88s. 🙁 Also, O/P trafos suitable for use with KT88s will put a serious dent into your bank balance.
IMO, you should get your feet wet by building a tubed preamp to drive a SS power amp. Do that or acquire reasonably efficient speakers that have a "flat" impedance curve. A PP KT88 amp is NOT a good 1st project.
I do believe a 12 watt amp should drive speakers over 94 db sensitivity, anyway Leak amps can work with very low impedances, I am not sure about the Stereo 20 but the TL12.1 have a 2 ohm tap!!!
Eli Duttman said:
Sorry typo, I've actually got B & W 602 S2's....
does that change anything or does the pre amp recomendation still stand????
iant187 said:
The spec's of 602S2s are somewhat more favorable. That @*&^%)# 4.3 Ohm impedance minimum is still present, but 2.83 V. of drive yield a 90 dB. SPL at 1 M. For tube amp purposes, your B&Ws are 6 Ohm nominal and 88 dB. efficient.
IF you can find a UK resident to tutor you, a variation on the "El Cheapo" theme will work. Under no circumstances should you attempt the 30 WPC "El Cheapo on steroids", without an advisor. The 7591 O/P tubes necessary require a B+ rail of about 450 V. Make a mistake around that and you are dead. Another issue is cost. Higher powered amps use costlier "iron". Sowter model UA21 O/P trafos will mate with 7591s. Sowter UA21s cost 145.83 UKP each plus (sic) VAT.
IMO, the "lie of the land" continues to suggest a preamp as the 1st DIY project.
Thanks I'll have a look into pre-amp kits.
Any suggestions? as a VERY basic starter (I'd really like to try out a valve amplifier, but I REALLY don't want to kill myself! cause that would suck!)
Any suggestions? as a VERY basic starter (I'd really like to try out a valve amplifier, but I REALLY don't want to kill myself! cause that would suck!)
Is WAD (World Audio Design) in UK still selling kits? If you want to try Hong Kong there is Brian Cherry of DiYHiFiSupply who also sells kits.
Read all the posts about safety, add a lot of common sense, and I think you'll be ok.
Read all the posts about safety, add a lot of common sense, and I think you'll be ok.
I don't see why everybody is so anti this amp. I was never a great Leak fan, but EL84s sounded very good (and Leak output transformers weren't catastrophic) Changing the output valves to octals involves major chassis work, the mains transformer isn't designed to give the right voltage (and changing that is a major mechanical job) and the output transformers were wound specifically for those valves (and on, and on - just about all the components would need changing)
So, the amp is not particularly powerful, and would seriously like higher effeciency speakers (we can get round to building those when you're happy with the amp) It will sound quite a bit louder than a transistor amp with the same rated output power (well, it will on transient-rich, continuous power low material, anyway) Valve output stages were not that critical of impedance curves; quite a few "vintage" loudspeakers were anything but flat, with certain curves giving serious problems to early transistor amps. The input sensitivity is dead simple to change; a couple of resistors, or an input pot, the easiest first exercise you could wish for.
Electrolytics will probably need replacing; not as simple as it at first appears, as their modern equivalents are physically smaller, and frequently the wiring to them is different, and requires attention.
The thing you can't do anything about; if the output transformer has got over hot, on some Leaks the wax tended to run out, and the output coils would "buzz", clearly audible on the speakers with critical material. I know of no way of re-impregnating the things, and if you have to change the output transformers you might as well start from scratch.
Dying of it? Most of my generation cut their teeth on valves, and the absolute number who died of them is very small. Burnt fingers, scars from exploding components, numerous shocks, evenspines collapsing due to trying to carry the more powerful ones, perhaps.
But a reasonable level of care and attention (which we didn't always give) you shouldn't be iat any great risk - and making a great sounding amp with valves is considerably easier and less critical than building its transistor equivalent.
So, the amp is not particularly powerful, and would seriously like higher effeciency speakers (we can get round to building those when you're happy with the amp) It will sound quite a bit louder than a transistor amp with the same rated output power (well, it will on transient-rich, continuous power low material, anyway) Valve output stages were not that critical of impedance curves; quite a few "vintage" loudspeakers were anything but flat, with certain curves giving serious problems to early transistor amps. The input sensitivity is dead simple to change; a couple of resistors, or an input pot, the easiest first exercise you could wish for.
Electrolytics will probably need replacing; not as simple as it at first appears, as their modern equivalents are physically smaller, and frequently the wiring to them is different, and requires attention.
The thing you can't do anything about; if the output transformer has got over hot, on some Leaks the wax tended to run out, and the output coils would "buzz", clearly audible on the speakers with critical material. I know of no way of re-impregnating the things, and if you have to change the output transformers you might as well start from scratch.
Dying of it? Most of my generation cut their teeth on valves, and the absolute number who died of them is very small. Burnt fingers, scars from exploding components, numerous shocks, evenspines collapsing due to trying to carry the more powerful ones, perhaps.
But a reasonable level of care and attention (which we didn't always give) you shouldn't be iat any great risk - and making a great sounding amp with valves is considerably easier and less critical than building its transistor equivalent.
iant187 said:
I have listened to this amp once, and it was a proper example of EL84 class. They have an English distributor if you search their website. I think it was relatively cheap. Maybe its better to look for something new with a good price like this one in general, and avoid the hassle. You don't sound like taking the DIY route for the time being.
chrispenycate, IMHO your entire post makes good sense. The only things I'd be inclined to disagree with concern the quality of the Leak OP transformers, which I would class as mediocre, and the overheating and wax-leaking problem, which I believe applied more to the power transformer.
I think some of the reasons behind people's dislike of (some) Leak products could be:
* The hype that surrounded Leak 40-50 years ago, which has since proved to be unwarranted;
* Leak's increasingly cost-conscious, penny-pinching attitude that led to a lowering of standards on their later Stereo 20, 50 and 60 tube amps;
* The excessive input sensitivity of Leak amps, coupled with the poor peerformance and quality of the accompanying Varislope preamps;
* The final demise of Leak, when they tried unsuccessfully to go SS with the Stereo 30 and Stereo 70.
I think some of the reasons behind people's dislike of (some) Leak products could be:
* The hype that surrounded Leak 40-50 years ago, which has since proved to be unwarranted;
* Leak's increasingly cost-conscious, penny-pinching attitude that led to a lowering of standards on their later Stereo 20, 50 and 60 tube amps;
* The excessive input sensitivity of Leak amps, coupled with the poor peerformance and quality of the accompanying Varislope preamps;
* The final demise of Leak, when they tried unsuccessfully to go SS with the Stereo 30 and Stereo 70.
Great thread. Of course the sensitivity isn't at all excessive if you use one of their preamps. Nevertheless, I have a Stereo 20 and I'd say it is okay, and can certianly be made better. I've kept it close to stock just for sentimental reasons, but don't listen to it regularly. If I were inclined to modify it, I'd absolutely keep it as an EL84 (great sounding tube!). I'd also likely go triode wired and eliminate the feedback and one stage of amplification. Those are major changes, well beyond the builder's original question, but I do think the amp would be excellent for this. Of course, you' have to change some of those lovely laced wiring harnesses in there too. Without all the feedback, I suspect hum would be a problem.
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